I've had this roblem on the Lionmaster class a as well as a challenger.
This is a "somewhat" common problem with Lionmaster locos. Turn over the locomotive, there is a screw recessed in the bottom of each set of drivers. This is the motormount screw.
It is likely loose or missing, and needs to be tightend or replaced.
On the most recent Lionmaster offerings Lionel added a retainer to keep the motor in place even if the screw came loose.