I dont own any WSMs. But from what I've heard and read they aren't as slick in feeding as your 'standard' cartridges. They say that this is due to the thick case body and sharp shoulder. Sounds logical. This may be the case especially with the .243 WSM and the .223WSSM.
I'm sure pushing that shoulder up against the front of the magazine feels like hitting a brick wall. Did you actually try cycling rounds from the mag to the chamber without being too tentative? Take it to the range or a safe area and try it. You may just not be used to this rifle/cartridge combo yet.