Cameras not recording back 31 days.

Hi had an issue with cameras set up to record continuously for 31 days to a nas drive direct to file server. I have a set of 3 cameras going back to a 2tb nas recording at 12fps with each cameras storage set to 500gb. I have exactly the same setup on another set of 3 cameras and these work fine , however on the cameras with the issue they only record back 7 days. Tried lowering the storage amount and increasing the storage amount but the problem persists. There are also another set of cameras same setup where they only go back 1 day. Any ideas appreciated.