When disk space starts to run low on the DVR set-top box your system will automatically start deleting older programming to make room to record new programming. You can lock your favorite recordings to prevent them from being accidentally or automatically deleted.
There may have been a conflict in your DVR Schedule. You can schedule three programs to record at once, however a set-top box (STB) will need to be tuned to one of the programs. If not, one of your shows will not record.