I looked for a past thread to use as reference but couldn't find one. There have been several.
Turn the locomotive over and observe the side to side play of the geared axles.
Sometimes excessive side play causes the gears to only mesh at half or less contact.
Trimming a bread clip down and making it round so it's not visible behind the wheel. You can insert it on the axle between the wheel and the truck frame on the opposite (no geared)side of the truck .
This will slide the axle over improving the gear mesh.
This could be why it's worse on turns, because the curves push the axle to one side.