3 Speed Gear box

[Espeefan]

If fine tuning your servo end points does not help, pop the shift linkage off the servo arm and try to manually shift through the gears. See how it feels. It might take a little (very little and slow) application of throttle, or rolling the truck by hand, to let the slots in the gears line up with then dogs. Same goes for shifting with a servo sometimes. If it feels like something is binding up, you'll have to pull the covers and observe where the hold up is.