I went through this when my first TM was new. Can't think of much that is more frustrating. Sometimes I had to pry the coupler open with the cheater bar for my springbars - not smart, of course. I wrote to Reese (who made my hitch) and to the company that manufactures the coupler on the trailer. No joy from either one (the coupler guys didn't even respond).
I finally figured out, by trial and error, and by lying down in the driveway and looking up into the coupler to see how it works, that the problem is that the trailer has moved forward, toward the tow vehicle and toward the hitch ball. The ball puts pressure on the movable portion of the coupler (use the Search tool on "pawl"), which is at the rear of the coupler cup. With pressure on it, the movable piece won't move. So put chocks in front of the trailer wheels, and pull the tow vehicle forward until the trailer just starts to climb the chocks. This moves the ball into the front of the coupler cup, and it unlatches, every time.
For what it's worth, this same situation can also prevent the coupler from closing (and gripping the ball) properly when you are hitching up. The fix is the same - chock the trailer into position, pull the tow vehicle ahead just a smidge, and the coupler will drop right down.
Bill
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