Something that might help, and something I learned from my grandfather, who does model railroad... he mixes white glue, water, and rubbing alcohol in a 40/40/20 ratio, respectively. He uses it to do his ballast on his tracks, but it also holds flock incredibly well ---- and you can spray it with a cheap dollar store spray bottle....... just pop the cap and spray water through to clear the pump after each use.....
The technique is to first use a glue/water mix, and then put your flock or ballast on, and then mist it with a touch of water, and then hit it with the glue/water/alcohol .... because the stuff is already a touch damp, the 3 way mix will soak in, and down through the flock layer, but it coats everything, and makes it stay nicely.... plus, you don't get that shiny glue look on the outside of the flock, nor the dry patches in the flock, that cause pieces to flake off after it's all dry and painted.
Nahuris