The IR receivers connected to your wallplates are powered by a DC voltage that the DA-550 inserts onto your coax line. That voltage can get shorted out if you connect anything (including a 75ohm terminator) to any of the outputs of your DA-550 withOUT going through a "DC block" or one of the wallplates.
If you've recently added something to your RF network, I'd try inserting a DC block (even Radio Shack
carries them) between the DA-550 and the thing you added.
Otherwise, you might try disconnecting everything from your DA-550's RF outputs, pressing its reset switch, and reconnecting only one of the outputs. If IR on the corresponding wallplate doesn't work, there may be a problem with your DA-550 -- time to contact ChannelPlus. If that IR receiver does work, I'd reconnect the other RF outputs one at a time until you find the problem.
One other thought: IR networks come in 5v and 12v flavors (don't ask me why, 'cause it's a huge source of confusion). Anyway, yours is a 12v IR system, so it's possible that connecting a 5v target might cause a problem.