You say 'soon after', was that minutes, hours, days or the whole 3 weeks? How long was it working right for? Have you checked signal strength and noise levels in UPStart? Also in UPStart, what happens when you do a network verify - does it find all your switches or do you have yellow boxes flashing all over and then many black (missing) switches? Did you always have the phase coupler installed, or did you add that after? What PIM are you using? I have had a PCS and HAI switch along with a Lutron dimmer and didn't see any issues. I'm using all SAI US2s now though.
If it worked properly for a while and now all of the sudden it doesn't, something has had to change. Is it always bad now or intermittent? It could be noise - check the meters built into UPStart. But having it work after a delay that long doesn't make sense. I think UPB will only retry 5 times.
Is this standalone, or is it being controlled by SW or a panel?
Sorry for so many questions, just trying to narrow it down...
Edit: Actually, come to think of it, I think I have a Lutron dimmer now on the same circuit, in the same box in fact as a 3way UPB dimmer. Everything works great except that particular switch does not respond at all to all on or all off commands, but works fine when addressed individually. I may have to pull that dimmer (was moved from somewhere else) and see if there is a change. If possible, do what Electron suggested and kill the circuit where the dimmer is if possible. Last resort you can always pull them out if needed - if we get to that point.