It highly depends, there can be numerous causes for such behavior.
Starting from some wrong data in database , orphaned recording files, bug(s) in VSM or SASD itself.
You didn't provide enough data even to start diagnosing, whether this is affecting cameras on all your mediaserevers, or only from particular one, is it somehow related to camera model or template ,
what type of clip, do you have enough space on mediapartition assigned for creating clips, what is status of this clips in SASD, do you see any error message.
Best - create TAC case, describe precisely problem, attach support reports from related mediaserver.
Also I'd consider upgrading. VSM 7.5 is very old, I know numerous bugs which has been fixed till VSM 7.11