Ok I assume everything fits fine on your models.
What IOS are they using?
The issue of tightness is not all time, modeless always. Issue with prime scan/Cerec and iTero
Might have to go out to the office and see what they are seeing. Tight at margin area? Tight proximals? Tight because they have undercuts you ain't blocking out? Tight because the shrinkage rates ain't right...bad oven, or element going? Tight at occlusal intaglio? Tight because of sharp edges of preps getting rounded digitally? Tons of reasons and causes is my point.
Many variables I agree.They remotely checked the mill, the numbers seem within limits.( milled cubic wax shape and measurement was fine) I am changing new all cutting toolings and surfacing touch plate with calibration, the sintering furnace calibration is within 3*c, blocking is automatically done by Exocad (I have to recheck that). I am pouring an imp. scanning / designing /milling two crowns with different thickness and starting point of cement spacer for comparison on the die.
I had a a discussion with the dentist on a particular prep, for a remake where the abutment in scan prep seems boxy in shape and in relation to adjacent teeth, which makes it difficult for insertion, and I suggested to taper prep to ease insertion.
Tightness seems internally and hi occlusion , and not on all cases which makes me wonder, I even suspected the shrink rate numbers on the puck!, I bring opposing .6 mm between adjusting antagonist and cutting crown occasionally. Never had this issue before.