Well, before you give up - how old are your magazine springs?
If they're kind of weak, that might be an issue too...
If you've been using them pretty steadily for years, it's possible that they're weak enough so that the gun will function without a can, but maybe it's just not got enough spring to get rounds up fast enough with a can on it? It's all I can really think of, since you've already verified the can works on a .40 Glock, and you've gone through a bunch of recoil spring weights...
Otherwise, that really sucks
The .40 should be a pretty good platform to suppress since 180grs are already subsonic..
Edited to add - if there's any three platforms I'd like to try in .40, it's a BHP, a 1911, and a full Sig 226, which is hopefully nicer than the compacts. I've got a 229 that's "meh" with .40 and .357 Sig, and I've got a USP in .40 that's a little bit nicer but still not great. And a G35 that again - not bad, but not something I'd take to the range for fun. I'm hoping a BHP or 1911 would break that round into the "fun" category for me..