Calibration

I find that I get immediate good readings when I calibrate my new sensor when blood is tested in the 90s mg/dl. When I cal. at a high glucose level (+180mg/dl) it might take most of three days before I'm satisfied with my meter readings.
Sorry if this has already been discussed.
Also, I'm recently finding after re-calibrating, after seven days, I'm only getting an additional 2 days, then the dreaded question marks. Anyone know if Dexcom is responsiblt for this?
All responces are appreciated.