To expand upon Bob's comments, your gage is perfectly adequate for inspection purposes, but does not appear to be adequate for SPC and possibly for capability studies.
The key question that Bob raised was are these parts representative of the expected process variation. If they are representation, then the gage is not adequate for SPC. If they are not representative, then you should repeat the study using representative parts. You can also substitute actual process variation from a capability study in for the study variation.
I noticed that your gage resolution is borderline inadequate for SPC. Correcting this might help a little, but don't expect major improvements. Appraiser C tends to measure smaller dimensions. Could they be applying greater pressure when measuring?