I had the same type issue when I had my F150 and used an EQ WD. The more washers the more it put the coupler in a bind. This might sound stupid but have you lube the coupler? I had to keep mine pretty well greased up for the under jaw to release. What I found I had to do most every time was make sure the TT was chocked, raise the tongue and when it was at the top and still caught, drive the truck forward an inch or so and that takes the pressure of the under jaw. I needed 6 washers but could only use 5 because of the binding.
