Welcome, by the way!
Atlas switches have non-powered frogs, so the wheels would lose power going over it, however you should be able to pickup power from the other wheels. Is this a diesel or steam locomotive. If it is a diesel, put it on another section of track and pick up one set of trucks while leaving the other on the track. You should still have power, then reverse to see if the other picks up power. If not, the wheels could be dirty or an electrical pickup could be having a problem.
If it is a steam engine, there is a possiblity that the engine is not long enough to reach over the switch.
Basically, check the cleanliness of the wheels and the pick-ups, and the track. Hope this helps.