The very first time I used my airbrush, it was perfect. Ever since then, the color beads. What am I doing wrong?? It beads on buttercream, MMF, and Satin Ice. I'm using the lowest psi on a Kopykake Airmaster.

theres 2 possiblities here, your spraying too heavily, too many coats at once so colour is accumilating before being able to dry giving the beaded appearance.
or and more likely, the low psi is giving a grainy appearance to the spray. turn the pressure up. i generally set my airmaster with the knob sitting somewhere between 10 and 12 oclock, this will give a finer mist.

Hmm...maybe I'm getting too close to the cake then? It's not the first problem. Just one pass of the airbrush will have it beading up. But if I turn it up, I feel like color's flying everywhere. I wonder if I should try spraying from further away at a higher psi.

is the surface of the cake shiny, wet or oily at all?

No more than buttercream ever is I suppose.