Hi Michelle,
I havent worked with that brand of powder, so this is pure speculation. I would try mixing a little less of the powder with the resin. You could also try mixing it as a stock solution, then adding it to a larger mix of resin. Alternatively, you can try to pour in layers. It may not fix the settling issue, but may make it less noticeable.
Have you contacted the seller? I would be curious to here what he/she suggests.