I get XM's like what you describe all the time. The reason for the ones I've had are because the pivot is a tad too loose. Try tightening the pivot & make sure the body screws are tight, let me know if it still sticks.
I always make sure to remove any grease that gets on the lock face as it will reduce friction & let the lockbar slide in farther when opened, potentially wedging it in. Since all XM's are carbidized from the factory you do not need the make up of Sharpies & pencil lead on the lock interface.
I did get a very old 3rd gen that's seen tons of use that the Carbidizing has worn off & it was a little sticky, after it was re carbidized it was butter to disengage.
Hope that helps.