Help a Noob with a diff problem

[daz75]

Stupidly bought a second hand car with another "near complete" one that ic ould build for my other lad (the joys of twins) I'm nearly there but i noticed the wheel weren't turning properly, so i decided to remove the diff to see if there was an issue.

I think the problem maybe the diff wasn't tight. So i'm trying to screw the diff screw into through the outdrives and into the nut in the nut holder on the other side, but it simply wont tighten. I've removed the nut and screw and done it without the outdirves and it tightens fine, the screw is the correct length but i just can't get it to work, any ideas much appreciated.

its a losi 22 by the way