Key unlocks car but has difficulty locking

[such01]

I have a problem with the key for my 2012 W212. It unlocks the car / boot normally, but I have to move around the car to find a "sweet spot" where it works when pressing the lock button.
This was a problem with my original key 3 years into my ownership, when it got to the stage where the key would no longer unlock the car. I switched keys and now, after 6 months of use, the same problem is occurring and locking remotely is a bit hit and miss. Batteries have been replaced on schedule and I have tried the reset procedure of holding the lock button and pressing the unlock 5 times etc...

Any ideas?

 

[such01]

actually just checked the key and the red light is present on the lock/boot button press but the unlock button needs to be held down for it to light up. The purple light at the end of the key is also showing correctly for each button press. Looks like a key problem. On my original key the red light is absent on all button presses but the purple light at the end shows for each button...