The vast majority of my switches are used Gargaves units. I do find that MOST of them need adjustment and fine tuning to operate well. If it is only one loco I'd suspect the loco.
But there are a few things to check on the switch. The points seem to be the number one issue I find. If they are loose they will flop over under weight and a WbB is heavy enough to do this. My fix is to make sudr the points at the frog end are supported with rail joiners. Some of my GG had these, others did not. I at times also place a thin piece of card stock under the points to make sure they are the same height as the running rails.
Good luck!
