I recently got the TF: Prime Arcee and starscream and they both have issues with ball joints. Arcee's shoulder ball appears to be too large for the socket and pops out with the slightest manipulation. I tried sanding it slightly, but i'm afraid I'd go too far. Starscream's leg joint has the opposite problem, the ball seems a tad small for the socket. Any tips for a novice to rectify the situation?
For Starscream, just apply a layer of superglue/clear nail polish to the ball, then let it dry, and then try the joint again. The superglue will form a layer to enlarge the ball joint. For Arcee, it's a common problem, for which I hear sanding has worked for some, while for others it hasn't seems like a mold issue. I'd say get an exchange.
I had the same problem with my Arcee. I took the Dremel to my Arcee's shoulder, and it works fine now!