After checking your motor plate is flat and the screws have washers under the heads...
Get some 600 grit wet 'n dry. Lightly sand the surface of your motor plate and the motor mounting plate. All you are trying to do is to make the surface matt, not remove the anodising. It should look lightly scratched all over. Now put the motor back.
Anodising is a low-friction surface. Sometimes, however hard you do up the screws the motor will shift. By taking the surface off it you increase the friction you can generate between the surfaces so the motor stays put in a crash.
Old 12th scale trick - works every time! HTH
