HELP..Welder Problem!

I have a Miller 135, .035 wire, 75/25 argon-co2 mix, i am welding on 3/16 steel.. heres my problem, usually it welds fine.. and everyones used to the sound it makes when they are welding, well mine will stop making that "crackling/sizzling" noise and u can just hear the gas coming out and the wire sort of drips in a way, like it hesitates to come out and drips into a puddle where my bead is at instead of sizzling and burning into the metal better, it is pissin me off..maybe someone has an idea.