Welcome to the forum!
A few quick questions to start with.
Do all your locos exhibit the same problem on all your switches, or just some/one?
Have you cleaned the track and the switches also? Use hardware store naphtha to clean track/switches/wheels/rollers.

If you have a voltmeter, check for proper voltage along the entire length of the center rails of the switches.
If you do not have a voltmeter, but have a lighted car, roll it slowly by hand over the switches and see if the light goes out at any spot on the switches. You can expect a loss of power as you traverse the gap between the wide center rail, and the normal rails. If your locomotives have 2 roller pickups, it could be possible that only 1 is receiving power, and as you traverse the gap, the locomotive will lose power and stop.

Next, run a locomotive alone through the switches very slowly until it stops. Turn off the power and get your head down on the rails with a flashlight in hand and physically observe exactly where the locomotive stopped, and the relation of pickup rollers to the rails. Some locomotives have very wide pickup rollers, and when going through the switches, the rollers bridge the center rail to the swivel rail and short out, cutting power and putting the locomotive in neutral.

Run these tests, and see what results you get.

Larry