Martin,
I don't really care for this set but have bought a few nonetheless, I have not experienced this problem with the arm guards so hopefully yours is just a fluke.
Heh, I remember so many crud toys--GI Joes whose backpacks wouldn't stay in. Star Wars figures who couldn't hold their guns...
What I hated worse about the G.I. Joe's was the way their feet always came off in their boots!!
Gepetto