You don't mention whether you've been successful at aligning Rachel when commencing an evening of observing. That topic is what most new Nexstar users first seek advice about. Are you beyond the alignment phase when your troubles start?
Tel is correct that when the scope has been aligned and is moving (GoTo'ing) to a target, pressing one of the four direction buttons will cause the slew to stop. Normally, after specifying a target and pressing "Enter," you stand back and wait for the scope to slew to the target. It's not clear what you mean by "the altitude stops working."