Yes, it says the SX2 will handle 2 3/4" shells, but if you download the Winchester catalog or view it online, you will find that the specs for the gun indicate that "light" loads for the 3.5" gun are 2.75", 1 1/8oz loads, not the even lighter 1 oz target loads. Yes, it's "in the small print", but it's still there. If you're having problems with 1 1/8oz loads, then you've got a valid gripe, but I'll hold to my opinion if you're having problems with 1 oz loads.
The catalog does say that the 3" gun will handle the 1 oz loads. Not much comfort, I'm sure, since you've already purchased the gun, but maybe if you follow the suggested "break-in" procedure suggested, the gun will handle the 1 oz loads eventually.