OK, this may not be helpful at all, but this sort of thing isn't unheard of. Actually the most common case of this, I'd say, is the player gets the first move command, like move left, and consantly moves left, without stoping...It's lame.
Somethings I've seen work, may not work, which is why it may not be helpful.
The first one I can think of is copying everything in that Event, and then cut and delet the Event compeltely. Make a whole new Event and paste it all in that one. Depending on the Event size (if you have many pages) it could take awhile, but it has to be a new Event to work, don't copy the Event and paste it. The second thing is more of a sugeestion. Are there other Events going on while the player Move Command Event is being used? If so, check these Events, you may have something hindering the Player'smovement or the Commands given in that Move Event. Check the timeing of these Events, it could be something simple...believe me. Make sure you don't have any Auto-Run Events. If you do, make sure you are using them properly, They can screw alot of stuff up. I hope I was helpful...sorry if I wasn't!