E34 3T alram problem

I have a 1995 518I with a 3t alarm (single button on three inch long fob) and over the last few days have had to get closer and closer to the car before the remote will open the doors. Now it's stopped working
altogether.
The obvious starting point was the battery in the fob so I changed this but still no joy. I then tried the spare fob which worked the other day but still nothing.
Could someone please tell me:
a: is this a common fault? b: do I need to disconnect the car battery and reset everything? Or c: do I need to reinitialise the fobs and if so, how do I do it?
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