Most girls just aren't that good at giving head. I'm uncircumsized too, and after I got head from a few girls I thought maybe that had something to do with it. I just couldn't come.
Then I hooked a girl who actually could give good head, and I realized that it's simply an issue of girls not knowing how to give good head.
The best head is the simplest. I've had girls who deep throat, and work all around the angles of my junk and pull out all sorts of crazy tricks, but it doesn't get the job done. The type of head you see in porn isn't really that great (my fb watches porn and hooking up with her made me realize how much of the stuff you see in porn isn't THAT great in real life).
She just needs to be sitting in a relaxed way, put the head in her mouth, tuck her lips back and apply a medium amount of pressure (suction) while she bobs her head up and down. She doesn't need to go down too far, just a little will get the job done. It helps if she massages your balls.
A girl giving you head, and you giving a girl head is similiar in one important way: consistency. If she does one thing for thirty seconds, then switches to another thing, over and over again, it'll never get the job done. Once she's bobbing up and down on it, tell her to just keep going till you're done.
It really is that simple, but most girls don't seem to have learned. Which is really the fault of the guys they date for not showing them.
Explain it to her the way I just explained it to you and she should be fine.