Is it one of those coin shape batteries? I've had loads of trouble with them.......

Nearly new ones failing. Maybe old stock = They do have limited shelf life & use by dates.

A loose fit in the key fob makes for unreliable contact. Very carefully "adjust" the contacts in fob or find a way of stopping the battery rattling about in the fob.

And my bike key fob won't work remmotely after a few days not in use. But hold it in the emergency start position* & it works straight away, and then continues to work for the rest of the day.

(*emergency start position: not sure if cars have this but its a point on the bikes bodywork close to the antenna where the fob will work even with a dud battery).