We've just fitted smart TRVs and since doing so we've noticed that the radiator in our bedroom is usually cold, even though according to our settings it should be heating up. It does heat up a little bit in the morning and evening, when we have a couple of our other radiators set to come on, but it doesn't heat up very much, and only really at the top. I tried turning all of our radiators on, and it heated up properly, but started cooling down as soon as I turned them back off.
I've checked that it's not the smart TRV by swapping them around, and I've contacted the manufacturers who confirm that the TRV is communicating with the boiler as it should. It's quite possible that this problem existed before we fitted them, but that we didn't notice until we had the data from the smart TRVs.
The affected radiator is on the first floor. We have working radiators on that floor and on the ground floor, and the boiler is in the loft. The radiator in question, even when it was working, was always one of the last to heat up after the heating came on.
I've seen questions about radiators that only turn on when the other radiators are off, but never one about one that only turn on when the others are on.
What could be happening with our radiator that would explain this behaviour, and what should we do (or get done) to fix it?