Error messages indicate that a regeneration failed. I've seen additional messages on occasion saying that some components coudn't be found. I have no idea why this would be since all the parts and the assemblies are in the same directory. I re-extracted the parts to a new directory, then moved the assemblies over and have recovered these drawings. The solution seems to be temporary. I've lost parts after the move, too.
For me, it is the motor that keeps disappearing. Not the only part, but it is the most consistent part. For other Team members, it is other parts. The difficulty in recovering the regeneration varies. This is very time consuming and frustrating. Everyone has moved away from the software so that we don't stall the project and end up unable to compete for lack of a built robot.
Since the software is provided free and is an academic version, we don't have support for the software. I am always torn between trying to fix it or forget it. Sorry PTC. I appreciate the availability of the software, but it isn't performing well.