I have heard from other technicians sometimes the valve spool can have a slight burr on it & jam.

BT say that valve for side shift & tilt in the main valve block isn't replaceable & there are NO replacement parts listed for it either.

I do know from a colleague it is possible to take the spool out & remove the burr to stop the spool jamming BUT be warned you risk ruining the complete valve block if you get it wrong / break it in the process.