I've had this happen on a few Fastrack switches on my layout b/c my benchwork isn't perfectly level across joints between modular sections. Some locos will pick the points and force them open, then one truck or set of drivers will start traveling the wrong way and trigger the non-derail feature. Shimming the switches or leveling out the benchwork has resolved the issues for me.
Also if the frog points don't sit tight against the outer rails when the switch is completely thrown one way or the other, you can bend them a little bit with a pair of pliers. However be very careful and gentle when doing this! I'm currently waiting for a replacement for one of my switches b/c I snapped the end of the frog rail trying to bend it - that frog rail is now disconnected and the switch is useless.
It's also possible that the non-derailing wiring is backwards. Should be easy to test - just set the switch to the through position, roll a car in from the out leg, and see if the switch throws to the through position when the car hits the isolated section. And vice versa.
