OK. Second post of the day and another problem.

I have a SC9000 security system which works fine as a security system but when I first set it up (April 07) it refused to act as a wireless transmitter. I installed a TM13 and everthing was OK.
Some months later I noticed that someone had turned the switch off on the TM13, but the MS13's around the house were still working. So I used the TM13 as a singles address plug module.
Recently the SC9000 has stopped acting as a wireless receiver and I am back to the TM13.

Any ideas about why this is happening? (And yes I have turned it off and back on again)