Our hub motors come in two widths, 75mm for narrorw forks, eg. Brompton bikes, and 100mm for most other bikes.
If it seems like your fork is too narrow for the hub motor, it is likely that you just need to swap over a couple of washers. Please check page 40 of the instruction manual for the full information, linked here.
By putting the torque washers on the inside, and easing the forks ever so slightly you should be able to solve the clearance issue:
If you are still struggling to fit the wheel, please email email@example.com with an image, spec of your bike and the kit that you ordered so we can assist you further.