DVR not recording two shows a a time.

I have the DVR set to record on a channel from 8:30 pm till 11pm, and another channel only at 9pm for an hour.  The DVR starts recording the show that starts at 8:58pm stops and records the show at 9pm which is an hour, but stops 30 min into it and switches back to the the original channel and records the next show that starts at 9:30pm. Essentially it is not letting me record two different shows at a time like it is supposed to, but switches back and forth between two channels.  I have reviewed the to do list and only two shows are set to record for the 9pm hour, there are no other conflicts and I have plenty of recording memory left. Why won't my DVR tape two shows at a time.