I had similar problems with Alt selector locking at 4200ft and wouldn't budge.
Then I noticed that in the FMS FPL page 1, right LSKr2 had 4232ft entered against the destination, I had just loaded a Flight Planner flight with only a Departure and Destination. The elevation for the departure airport was correct but the elevation for the destination was 4000ft too high.
Once I had changed this to my cruise ALT everything worked perfectly and after selecting an arrival, the FMS actually did a descent during my approach to YMES changing my ALT selector to the next height restriction.
I wasn't paying a lot of attention at the time as I just wanted to unlock my ALT selector, but I will investigate it more on my next flight.