If all the components are as described, then it has to be down to alignment and set-up
I'd suggest taking it apart and trying again from the beginning. You are right to be cautious about running it with the belt slippage - it will cause damage if you continue to do so.
Slacken the retaining bolt, move the motor towards the centre, re-fit the mod bracket, ensuring the "wall" is well away from the leading edge of the gearbox, push the motor back outwards as far as it will go and retighten the retaining bolt.
The belt should be a tight fit, to the point where you are having to rotate the drive cogs by hand to assist getting it on.
If all this fails then we are at the limits of what we can do over the internet