Thanks for the suggestion and link. Some of the codes worked--but unfortunately, it was again only for a one single command or click (i.e. holding down last key didn't continue sending the command), so not sure this will work.
But I did discover something else. If I press a key using Xfinity remote directed to Aux (receiver), for example VOL+, it will send the command one time (like I mentioned before), but if I then press a command on the remote that controls another function--cable box or TV--and then press the volume button again, it will take the command. So if I rotate between VOL+ and any button not assigned to the receiver, it "works" (sort of). I have 2 different Xfinity remotes and it works this way with both of them, so I doubt it's a broken Xfinity remote.
But I don't know if the problem is related to the Xfinity remote or the Denon receiver. It's as if the remote will not send more than one signal to the Denon receiver consecutively without changing devices. But that sounds quite odd as I never had any problems with these remotes and my Sony receiver or controlling the TV. If I can't figure this out with the Xfinity remote, I suppose I could always try a Universal remote, but not sure if that would solve the problem or if the problem is related to the Denon receiver. It seems to be an odd problem I know, but any other ideas?