I'd say it's a combination of a very tight new mag spring and possibly not pulling the slide all the way back when chambering the first round. If it happens again, try loading 6 rounds in the 7 round mag and see if it happens again. I'm thinking you're getting a little too much resistance when stripping the first round. Yes, you should be able to put 7 in a 7rd mag, and no, you shouldn't have to break in the mags. But it's not a perfect world, and trying 6 instead of 7 for a while might help isolate the issue.
A "dead trigger" feel is not too likely to be ammo related, and for what it's worth, I have 100's of rounds of WWB bulk through my Shield with no issues at all.