Many suggestions.
What make and model of loco? What track switch make and model? What control/power system?
Have you measured the distance between the rollers, and then measured the equivalent distance on the switch, after determining exactly where the power failure occurs?
Have you taken your voltmeter and slowly and carefully tested the power rails in the switch to see if there are unsuspected dead spots?
Have you searched this or other forums to read what other guys have experienced and how they corrected the problem?