I used to live in Sutton coldfield near some high rise flats.
Used to get this issue as well, sometimes it would work other times it was impossible to get in.
The cause was a mobile phone mast on top of the flats.
It only affected certain frequencies which included car alarm fobs
,wireless doorbells etc...Eventually it got so bad that we had an inpsector out who measuered the microwave signal in the street and it was way over the legal level.
T-mobile
(at the time) were told to sort it out and sure enough that sorted it.If your struggling to make your fob heard, have a look around.
... if you can see a mast chances are its noisier than your key fob!