Having issues with key fob.

I'll go for a drive, unlock the car with the fob. Go for a drive and get to my destination, get out shut the door and I wont be able to lock the car. If I go into the car and shut the door then It will let me lock it all of a sudden. Or if I leave the car unlocked and walk off and come back 10 mins ill be able to lock it straightaway.

I send the key to be looked at, microswitches were fine and some bits got soldered but it's made no difference. Battery was changed too.

When I can't lock the car, the red light is flashing on the remote but on the car the red light isn't flashing. Obviously when it is working both red lights are flashing as they should.

So I think I can rule out the remote being the issue. What else could cause this? I can't see anything else that is consistent.