Not sure if your engine is similar to the one in this service document, page 1-53 (document page 84)
https://www.lionelsupport.com/media/supplements/Supplement1to9/1Complete.pdfIf it is, then the motor truck parts view is on page 15-60 (document page 409)
The most likely problem is the worm wheel slipping on the axle.
If this is not your locomotive, then please upload pictures of the motor and views of the truck showing the gearing.
Larry