A couple of possible problems exist with all key fobs, and mostly to do with batteries…
If the fob has not been used for a few months the contacts to the battery can tarnish.
Operation of squeezing a button produces the tiniest wipe of the fingers as the case flexes slightly. The wipe maintains connection.
If the tarnish is thin a few presses might restore operation.
Also some makes of battery use a glossy shiny metal outer that does not make good contact with socket contact metals! Wrong chrome alloy or something, maybe a chemist or metallurgist can give a proper answer.
However, a squirt of Servisol Super 10 onto a tissue/kitchen-roll etc which is then gently wiped across the contact fingers and also the battery case seems to make all the difference.
So far (touch wood) it has always restored recalcitrant fobs, providing a flat fob battery was not the problem.
One other thing to be aware of, an old-stock “new battery” might have enough volts but it might also lack sufficient “cranking” current to power the fob.
Always check expiry dates!
If in doubt, quickly dab a torch bulb across the battery and see if it produces a glimmer, if so then it’s probably OK.
I forget what the rated Short-Circuit current is for our fob batteries, but I expect 50mA will be ample when tested on the 1Amp range of most multi-meters. 1mA or less is definitely dead. I’m out of stock of any batteries right now so cannot do a test.
Maybe someone can do a quick dab-test with a meter on a new button cell battery and report the result.