I amended an existing program (7:00PM to 8:00PM) to 6:00PM-8:00PM by changing the 7:00 PM to 6:00PM so that now it should have recorded 2 hours of programming. Instead it just recored the first hour. Since this programming is Monday-Friday, I checked the timer and it read 6:00PM to 8:00PM. Not exactly the same issue as yours, but similar. Oh, the DVDR is a Magnavox (Philips) MRV640.
You said that you "cleared" the old recording (which means delete it to me). Did you really do this or did you "amend" it under 'timer listing' by altering it.