I tried some plywood pans on belisle 120s and couldn't get them to fire without a LOT of pressure. .
I suspect you were setting the pan to level. In order to function properly a pan trigger must angle up about 20 degrees over the back-trap jaw with the trigger dog pointing to the box entrance. If the pan is set to level it may hang up on the box floor and not release the dog or take extra pressure to do so.