Cameras stop recording when it should

Repetitively not recording any video for a short time when it should. Attempting to document improper activity at a specific itime. Records before and records after. Only proof is I physically witnessed to what should be recorded but isn't. Yes, I have checked my equipment for working properly. Three cameras all do not record for about short time during the same time frame...sometimes it is a few minutes up to around thirty minutes then it goes back to normal recording. So what is being used to block the recording.