I have also been having this problem... I have reported it before but no solution has been forthcoming to date, so thought I'd add my reply here.
I have the DCX3510-M model HDPVR.
This has happened randomly several times now, and there's no discernible pattern. It can happen when:
1. The show is the highest priority in my priority list.
2. There are no conflicting shows scheduled at the same time.
3. With a show in a series that is set to record 'all episodes'
4. Randomly on a series that has recorded several episodes but now somehow skipped one.
I have tried the following to no effect to save time with solutions since these are the things tech support seems to default to:
1. unplugging the PVR and plugging it back in.
2. unplugging the PVR and removing the cable as well and plugging both back in.
3. deleting the series and re-creating it.
4. deleting the series, unplugging the pvr, and then re-creating the series.
Tonight, for example, the new True Blood aired on HBO at 9:00 PM as usual. My series recording didn't record it. I noticed when I went to watch it, and have now set up a single recording for 11:00 PM tonight.
Edit: Below is the thread from the last time this happened....interestingly, it's actually the same series.