Well since it's been a year, they might not take it back, dunno. But, having said that, are you sure the follower is wider, it is the mag well slightly smaller up front? Sounds like a silly question, BUT, there is a very real possibility the mag got a little squished somehow, not unheard of. Can you measure it? If it just a minor difference, just tweak the magazine a bit to make it fit.

Also, make sure the follower is going in the correct way, ie, front towards the front. They are easy to mix up. The tolerances on these things are not precise at all, so unless there is a 1/4" difference or worse it should be no problem "adjusting" it to fit. These things are really a pretty flimsy metal, but they are not exposed so they do not need to be heavy duty.

I just bought a magnum setup for my long action and honestly, I went back to the '06 setup. It took some minor tweaking and not only does it feed the 338Wm well, it feeds the '06 brass better than before. The new magnum mag was too narrow, to allow the brass to slide through it. The new setup was for blind mag, which I WAS going to convert to, but after tweaking my detachable mag setup, I changed my mind.

I decided not to adjust the blind mad parts, until I decided I would really need them.