Does an empty magazine work? If so, probably just heavy spring tension to push against when the top round is hitting the slide from below. Locking the slide back first then inserting a magazine solves this problem, or just a decently hard tap on the baseplate works for mine. The 12 or 15 round magazines don't seem to be quite as hard as there's a little more wiggle room in the longer magazines.

Sometimes a new mags spring needs to be worked in. Try fully loading it and let it sit for a week.

Either you have a real strong spring or your first round is pushing on the bottom of the barrel. Try inserting magazine with the slide locked back. If it locks try taking one round out, let the slide forward, try inserting magazine. If it locks you have a spring that is too strong and needs to set. Or the spring is bottoming out and doesn't have room to compress more in magazines body. You'd either need a different magazine or find a flat wound spring to replace the OEM with.

Hi Richard and welcome. Some new magazines have high tension springs that need to stay loaded for a while to be able to fully seat on a closed slide. This will be the case if the same loaded magazine seats and locks in on an open slide. If it does not lock in on an open slide, then the mag is likely out of specification. Cheers and stay safe!