are you putting it on the "side" of your arm under the delt and tri, or are you putting on the "back" of your arm?
I am fairly lean too, but the arms is one of my favorite spots. On the back side though. There is no "motion" of the arm then to bend the canula, other than if you sleep on it and push it "into" your arm (which could then hit muscle, and cause bruising/bloody canula, which happens to me from time to time, so I will change that those if they start bothering me).
I also agree that the flank (back side, love handle area, even if you're skinny) is a great place. I get some of my best absorption there (and/or fewest "issues"). I'm a stomach sleeper or a side-sleeper, and it's far enough to the back that I don't notice it when on my side, and it's far enough down (inferior) on my back that it's in the lordotic curvature so when sitting at my desk I don't notice it in my chair either.
And one of my newest sites I use in every rotation is my pec. You'd think it has even less fat there...saw a picture of Kris Freeman (the Olympic skier) w/ it there, and I had to try it. Works GREAT. Point the canula up. Flex your pec. Make a mental note of where the top "edge" of your muscle is. Relax your chest, and place the canula insertion point about 3/8" above that "line" on your chest. Perfect location. I always bend over and roll my shoulders forward (and pinch up the pod) when inserting there, just to make sure I don't hit muscle on insertion. Haven't had any major issues there--it's lean enough I occasionally get the same soreness/bloody canula, so I will just change it then. But otherwise, works great!