This is as I remember (3 day limit), although someone else recently said that Dexcom-USA has replaced HIS failing Sensors on later days.
Dee, have you used ALL of the insertion tricks which are posted in various places around here?
#1, Use Smith+Nephew “Skin-Prep” before attaching the Sensor pad to your skin. (Not “IV-Prep”, Skin Prep is different – and made for exactly this problem.) This is what I use. You don’t have to leave a donut hole for the Sensor, you just wipe the entire area. The thin layer of skin-prep filler left behind has no effect on Sensor accuracy or lifespan.
Rather than let this stuff dry by leaving it in air, you should use a hair dryer pointed upwards into your target (Not too hot, and not too close.) Skin Prep starts out really thin and runny, and will flow away from the TOP of your site under gravity – unless you use the wind of a hair dryer to push it back up where it’s needed. Adhesive failure nearly always starts from the top, so that’s the part of the pad which needs special attention. It dries fast, you need to have the hair dryer running and ready to grab BEFORE you do the wipe. Wipe and dry completely, twice, but without over-drying. If you see big cracks in the Skin-Prep surface, it’s too dry: re-wipe and dry it less. (Lots of really, really tiny cracks is perfect.)
Or #1A, use a better-sticking “helper pad” underneath the Dexcom pad. (Opsite Flexifix is very popular for this, bought in 2" rolls within the USA. IV-3000 is also used.) You need to cut a small hole in the middle as a target for the insertion, the Sensor can’t always punch through successfully.
#2: When you set the Sensor assembly down, don’t press the adhesive pad, or the Transmitter Housing assembly into your skin right away- that causes folds and ridges in the adhesive pad. Just set it down softly first, and push the pad to your skin with the lightest possible pressure.
#3: After it’s down, remove the safety clip from the “shooter”. Use the rounded edge of the safety clip to press the neighboring pad fabric and skin down, not too hard, WHILE your press different sides of the Transmitter Housing assembly into your skin. (You still haven’t shot in the Sensor.) The idea is that you need to keep the adjacent fabric nearly level while you are pushing the Transmitter Assembly into your skin. If you press in the Transmitter Assembly without pressing down adjacent skin, the fabric and adhesive gets badly stretched at the edge of the assembly. This is not good, so keep that edge as level as possible.
#4: (You still haven’t shot in the Sensor). For the rest of pad, well away from the clip assembly, you can use a fingernail or keep using the safety clip. But don’t DRAG either of these across the fabric: Press in, then raise your tool or finger back up above the fabric before moving to another spot and repeating the press-in job.
#5: Place your thumb and MIDDLE (longest) finger on the clipping pincers which open up to remove the insertion plunger, but don’t press this thumb and finger together yet. Using the other hand, Insert the plunger smoothly, and don’t let go. NOW push your thumb and finger together to open the clips. It should come off very easily. (If you keep grabbing and letting go of the insertion plunger, OR those pincer clips, things move around. After the Sensor wire has been inserted, you need to avoid ANY movements of the Transmitter assembly!) Do NOT let go of the clipping pincers, but you can relax and let them close.
#6: With your other hand, set the now-removed inserter down. Pick up the Transmitter and fit it into the end of the Housing. Note that by using your Middle finger on one of the pincers, your index finger is free to hold the transmitter in place. 
Just let go of the Transmitter with that “other” hand, and grab at the tilting “Press-The-Transmitter-In” part of the assembly.
#7: Softly tilt the “Press-The-Transmitter-Down” arm down to the top of the Transmitter, where it replaces the index finger from your other hand. Now re-open the clip pincers, just slightly, and tilt the “Press-The-Transmitter-Down” arm all the way- to push the Transmitter in place. There’s two very important techniques you need at this step: First, use only a tilting motion: the Transmitter Assembly as a whole must not be pressed into your skin. And second, be absolutely sure that you hear TWO clicks, not just one. The relax the pincer clips- but don’t let go of them yet. (If you fail to get both corners of the Transmitter underneath the “pincers”, it will keep losing electrical contact. It will also slide over the contacts attached to the Sensor wire, causing motion by the wire. Motion by the wire in your skin ruins Sensor accuracy and lifespan; it also creates itching and pain.)
#8: After you’ve heard both clicks, also verify that that both corners are underneath using your eyes. (Visual check.) Here comes the last trick: Your thumb and middle finger are still on the pincer clips arms (but only loosely). Slide them towards the ends of the clips near the Tilting “Press-The-Transmitter-Down” arm, and grab on tight before you twist the Tilting Arm completely off. Use your grip to prevent the Sensor assembly from twisting while you break of the “Tilting Arm”. (Any motion by the Transmitter Assembly after you’ve shot in the wire is bad…)
You also want to be using a site which doesn’t s-t-r-e-t-ch the Sensor pad fabric during your daily activities. Frontal abs are no good for me, they move too much. (I use love handles.) Just breathe and watch in, or bend your arm… you might find a back-of-the-triceps site which shows relatively little motion. Lots of women, and a few guys too, find upper butt cheeks to be excellent sites as well.
Six days is absolutely sucky performance. (If mine make it into day two without showing problems ALREADY, then they’re always going to last at least two weeks.) You might not be able to get 14-17 days, as I do, but nearly every adult can get at least 10. You probably just need to find YOUR best site(s), and use all the insertion tricks which Dexcom’s manual doesn’t describe.
Just let go of the Transmitter with that “other” hand, and grab at the tilting “Press-The-Transmitter-In” part of the assembly.