Problem With Planner?

Hi

I seem to be having a problem with my planner on my hd box. If I record something and watch it when I turn the programme off even if iv'e not finished watching it a lot of time the box will just delete it without me telling it to. It's not because the box is full because I have room on it and everything else seems to work ok. So just wondering what could be wrong?

Any help would be appricated.