Check the battery contacts in the DS10A. I've had problems with them not making a good enough contact. They initially worked fine & like yours failed to report in to the PS561. The one I had a problem with was in a bathroom also. Could be condensation related. Stretch the spring out a little.
On another note, I had a problems with one or 2 sensors not checking in after they were installed OK. I have about 30 sensors in all. I would have to press the test button on these & then they would be OK. Then in a day or 2 some other sensors would not report in. Now, I can't verify this but what I think was happening is that I was getting RF sensor collisions when they would report into the console. In my activity monitor, I could see that a lot of my sensors were reporting in around the same time. I don't know how often they report in or the time interval but I suspect that pushing the test button on the problem sensor resets its' internal "report In" clock and offsets it to reporting in at a different time then the others. In time (about 2 weeks), my security system settled down so to speak and I very rarely have this problem anymore. If you have installed many systems in a small area or have many sensors in one install, this could be your problem.