You might need to change the diff screw/nut. Sounds like it might have stripped out. It should be able to tighten down.
Being new to 1/10th scale electric and ball differentials, you might want to download the manual off of Losi's website. They have all versions of the XXX-T manuals available there. It is a well put together book and in the back they also explain how to adjust both the diff and slipper.
Jerome
