In the picture below hopefully you can see that at the top of each of the "teeth" there is a little ridge that stands higher.
This is due to the fact that the belt has a little groove in the valley between the teeth. I guess this improves flexibility of the belt.

For the pulley this is a problem as not all belts have it, and this little ridge holds a conventional belt off the pulley enough so that it does not engage properly.

The solution is to simply turn the ridges off. Once this was done my new belt engages perfectly.
This was easy as my epoxy is filled with metal not sand- machining is practical and went without a hitch.