The collet in an APC 7mm connector should be able to turn and should slightly protrude from the center housing by a few thousands of an inch. If you slightly press it, it should go flush and return to original position upon release. Also, under a magnifying glass it should have no damage to the measuring surface. If it does or is stuck in the center connector it should be replaced.
Yes, a defective collet will have effect on the quality of your measurements.
Thank you very much for your informative replies, I really appreciate your feedback.
Dr. Dunsmore, you were about to write something in reference to the outer ring but you stopped writing abruptly, I am a little interested in what you had to say. If you have the time, can you elaborate on what you were about to say?
What Dr. Joel was trying to explain is the threaded ring on the outside of the connector should move freely. By turning the outer nut in one direction you should be able to have the ring protrude from the measurement base. Turning the nut the other direction the ring should recess behind the measurement plane. The nut should move freely at all times. This was designed to allow the other connector which either has the ring protruding or not to mate. If you have another APC-7 connector you should see this work very smoothly.
If yours doesn't the connector needs replacement as the ring may be bent or threads damage. This would pass damage to another connector that is mated to it.