I unboxed my Universe Piranacon to help you!

Try this: because Seawing (unlike the other limbs) doesn't use his head as the connector, try pushing his peg fully into Snaptrap's socket. You notice that Seawing can click in once (leaving him floppy) but straigten up and push again so that Seawing's body is almost flush with Snaptrap's socket. You might have to wriggle and force it a little but the peg will go in further. You should endup with a 3mm gap between Seawing's body and Snaptrap's shoulder. (instead of the 8mm gap you may have had before).

If you're already doing this and he's still floppy then sorry, I'm out of tricks! Might be a problem with Snaptrap's socket?