XR15 remote no longer changes channel through wall

I have one comcast box and 3 remotes. I used to be able to use both remotes to change the channel through a thin wall. Now only one will do it, the other two won't (have a splitter so one box for 2 tv's - since I only watch one tv at a time it doesn't matter the station will be the same on both tv's).

Comcast forced me to change out my box and this is when it stopped working. So then the changed it out again (tech was here) and same problem. I asked for my old box back and he said he can't do that. He traded out the remotes that now weren't working with the remotes that came with the box. That made no difference.

Is this a box problem, a remote problem or both? Since one remote still works that way I am thinking it is a remote problem? Is there any wqy to fix this without having to drive out to the xfinity store? I have already reset the remotes. Not sure what else to try.

Thanks in advance