I have had this problem in the past but not for a long time. Then it just happened. I don't know why but here's what happened, and what I did to fix it.
I was watching TV this morning, flipping channels and finally settled on something. After watching for a good 30 mins, I tried to pause something on Cinemax (I have paused this channel before with no issues so I knew something was wrong). I flipped around to other channels to see if any of them would pause, but they wouldn't either. I finally just powered the Comcast box off, waited a few seconds and turned it back on. Obviously signals had been crossed because, unlike normal, when I powered it back on, it was on a different channel. That never happens. Usually when I turn it back on, it goes right back to the channel it was on before. I tried to pause the new channel and it worked fine. Put it back on the Cinemax channel and it also paused just fine.
I don't know, or how, but clearly at times the signals can get confused (?) for lack of a better word. Powering it down so it clears the memory seems to work for this. At least it does for me.