Our 2006 Monterey doesn't use the pos-a-lock described by Jeremy. A previous thread this fall discussed issues with the simple scissor hinge mechanism, like ours, that Jeffrey probably has. Loose or missing screws on the bar's mountings, either on the door or the door frame, can affect operation, as can a loose "central pivot rivet", or a worn stop flange. The entry door is subject to a lot of use and wear, and a little abuse when inadvertently slammed. The points mentioned, when faulty, allow for sloppiness, and a detent (near the "rivet") that won't set to hold the door open.
Joel