Don't know whether it's relevant or not, but it sometimes happens to me. Twin tuner Freesat Humax box here in France. You can only record on two channels, and also watch a recording, else you have to switch it off. What seems to happen to me is that i am recording so much, the program start and end times tend to overlap a tad so it's possible that there are three channels recording at once and one just gets cut. I therefore juggle with recordings using the +1hr channels.
I believe that Freeview boxes can now get up to four tuners(?) but not so on Freesat.
Unless you have more than two tuners, changing the start and end times mean diddly squat!
Regards