Compressed air from an airline can sometimes un-jam the thing without contaminating the belt material. Yes they are positional in that on certain slopes they will refuse to release or sometimes retract. Sounds like yours has this issue fairly regularly though. I'd take Arwyns advice and whip it out then send it for service and like DaveW said, don't take it apart they clearly have spring tensioning tools for this job! It's usually dusty muck that becomes sticky and causes the ball to stick in the locked position. Just check that the belt hasn't twisted and is sticking in the guide that sits at the top of the seat back although I assume you have been tugging at it closer to the reel!