Remote key working intermittently

[SamW]

I've been searching some information related to this but have not found the definite answer yet.

So, I got two "diamond" shaped remote keys for my X5, let's call them A and B. When I bought the car, the key A was the main key and it worked just perfectly. The key B however, couldn't be used to open or lock doors remotely.

Then maybe two months into the ownership, the key A started to require a several pushes of buttons to open or lock the doors and it got worse little by little to the point I was almost forced to use the door lock manually to open and lock the doors. I thought the battery was going bad so at this point I got the key A checked by a company specializing in car keys. It turned out the battery was fine, micro switch buttons were fine but the key wasn't sending a signal. They added some solder to the edge of the circuit board and reported that all is fine now. The key A seemed to work a little bit better after this but still required several pushes to operate the lock remotely.

I then got the key B check in order to see if problem is in the car itself or if the key A would be defective in some other way that the shop couldn't test. The key B was tested but its battery and micro switches were also in shape. I then synched the key B with the car and it now works for opening and locking the doors but it still usually requires several pushes of buttons, not so many than initially though. Locking the doors with the button inside the car works fine by the way.

So all this makes me think that there are two options now: either the both keys are someway defective in which the shop couldn't test or the problem is in the car itself. I bet it's the latter, so where should I start looking? I bet there is some kind of a receiver somewhere or maybe the wiring? Anyone faced a similar issue?