Thanks! I don't understand this one "If you can wiggle the worm gear around, that's your problem." it doesn't wiggle when chassis & wheel base are together, but wiggles when separate

The wheels won't turn but will turn fine when I press the chassis with the bottom of engine, it seems that the worm gear box doesn't engage fully with the gear on the 3rd wheelset. I already tighten the screws but still.. Is there anyway to fix it? Thanks!

I did the following tests--

- wheels do not turn freely, so it seems the main drive gears are fine
- belt and worm spins fine when chassis is dissembled from the driver set
- One plastic separator between the 2 split metals is missing one leg, but I was able to screw in the hole