I only have a siphone style blaster, but I had the exact same issue.
I removed the front half of the gun and pulled the venturi - I noticed the clearance from the tip of the venturi to the inside of the Nozzle was very tight. Didn't let media flow very well at all.
So I ground a slight chamfer on the front of the jet and then added a couple washers and an o ring as a spacer to push the venturi back out of the Nozzle a bit. This left more room for media to flow, and flow it did! Was able to get the walnut media to feed consistently. Coal slag was even better than the walnut though, so I'm sticking with that.
