We had a similar problem with the front washers losing pressure on a MCS
I took the washer bottle out (take off the wheel, then carefully remove the arch lining).
Discovered there was a thick black rubbery deposit on the motor inside the tank.
Disconnected the motor from the tank and flushed it through and all is good.
I was advised that this is a common issue with BMWs and they "recommend" only using their own washer detergent.
A low skill but messy job. Be careful if you disconnect the motor power and flush the tank from the top you can easily spout water into the electrical plug!