The 135 is suppose to change wire feed while changing voltage too. It's called wire speed tracking. So when you get the weld settings right for a particular thickness, if you want to change to a different thickness, you can just change voltage and the wire will track with it and keep you near.
There are too many things that can make it weld "bad"
bad wire
polarity wrong
bad liner in gun
bad gas quality
poor contact tips
cut in gun liner...

A-