Apple watch allows third parties like Dexcom to make “complications” aka apps for the watch.
The complications are the view of the app you can put on a watch face.
My understanding is apple is very restrictive in how often apps can request to update values on the watch.
Dexcom can request to update the watch but Apple is only going to do it so many times per-hour to conserve battery. I think Dexcom is likely doing their best to be accurate by displaying “–”. It seems to do that when the watch has not received and update for more than 15-20 minutes. That makes sense because you don’t want to make decisions based on old readings.
There could be a bug in Dexcom’s watch complication but its mostly and issue of Apple being overly protective to the battery of the watch.
I wish they would allow me to approve Dexcom to update as often as needed in trade for battery life.
This is very technical but there is some discussion about watch complications in general here.
They briefly talk about a watch app exhausting its budget for a given time period.
I’m a software developer so I do understand why these restrictions are in place but I still wish they provided me some way to relax it per-app.
Still I’ve been using the watch to track my sugar and prefer it to taking out my phone. When in meetings or talking to someone at work checking the watch is much less intrusive.
Another trick I have is once the Dexcom app is open on the watch if I rotate my wrist away (screen off), then back toward myself the Dexcom app seems to update 95% of the time.