If the air flow is to high, the sand will separate from mix.
I don't have a great understanding of CFM's but most spray guns suggest 7 cfm at 25 to 40 psi
Can you tell us your mix design?
Blow-back is common and there are ways to deal with it. Not having Acrylic in your mix can increase the blow-back as your mix is not as sticky and won't consolidate as well. Knowing your mix may help....
Another thing that can create blow back is the sand size. Finer sand causes more blow back.