Maybe the metal part inside the wheel is wrong way up ?
Please, help US in helping you, and make some pictures of the sitution.
Belt: I feed it round the large coge, cause I can rotate it anticlockwise free, and then I put it on the small one. Then I rotate the large one with my right hand, the left hand thumb feeds the belt on the small one. Once it goes there completeley round for 2 mm depths I won. Just turn and turn the large one, see that it won't slip of the small one and you're done. Finally, after 10 turns you can see how it would run on the small one - I mean it's overlap / non-overlapping area. The non- o is to be MINIMAL, cause the belt will tear then where the borderline is. So you goal is to adjust, again, by washers'n spacers, the large wheel, so that the belt rides FULLY on the small one. If so, AND the gearbox is supported by the fence, and the belt has maximum tension, THEN it is 100%.
Carsten