Sounds like a mechanical problem with the trimmer.
The error appears with print jobs, we do virtually no copying on this machine. I am unable to determine a pattern. We can print a short run of 100 booklets and 10 of the 100 will not trim. It can be random, 2 or 3 in a series, or perhaps all 10 and the rest are normal. However, we have had cases where we will run a job of 50 one day and run the same job a day or two later and the machine does not trim the second run, even when the settings, stock, size, etc. are the same. I have had multiple service technicians atttempt to diagnose this problem.
Not knowing all the facts, I would then try to determine if it is a rip or machine issue by running copy jobs with the trimmer. I'm not sure what 'happens periodically' means. You have to narrow it down to either the machine, the rip, the trimmer or files.
The software has been updated to the newest version. This has been an ongoing issue since installation of the machine over 6 months ago.
Make sure the C75 software has been upgraded. The riggers do this at install but sometimes miss it. A tech can check to see if it was done and that all the platforms are at the correct levels.
Periodically the trim function on the square-fold trimmer attached to our C75 fails to work. It will faily to trim 5-10 books in the middle of a print run and report no errors, trmming books before and after. In some cases it will fail to trim an entire using the same settings, paper, document size, etc. for a job that has been previously run. The techs have looked at this on site, and seen this error in operation with no resolution.