Another AC question?

[myyamr7]

I recently had my AC charged up, new compressor, all new lines, and I found a local shop with some R12 to charge it. The AC works and is ice cold, but if I drive for about an hour or so with it on, it starts to get a little warm inside the truck. I opened the hood when I got home, and the big return line had a good layer of frost/ice on it. By looking at the water flowing from my ac/heater box shortly after, I'm getting the part inside there was frozen as well?

Any ideas what it is before I bring it back to the guy?
Thanks