Try this:
Make sure you keep firearm pointed in safe direction, finger off trigger,and preferebly do this at a range.
1) Load Magazine
2) Lock Slide back on gun
3) Insert Magazine
4) Either a) press down on slide release and let slide close that way or b) pull slide to rear and let go.
See if it chambers this way. Some rounds will get caught up if you pull back and close the slide slowly.