Try locking the slide back before inserting the magazine. Then insert the magazine and drop the slide. The top round of a full magazine is being forced upward pretty hard and is probably dragging on the bottom of the slide as you try to rack it.
If you are talking about racking to load a round, I agree with Smith Shooter. Leaving the slide locked back never worked on my first 9mm Shield. I had no problem racking it to load a round. But, it was very hard to lock back for cleaning without a mag inserted. Sold it. But, now have a 9mm Plus with no issues. Still not easy, but very manageable. Now, while firing I have no issue racking to load a round. Try grasping the slide firmly and try pushing the frame forward more than pulling the slide back. Just release quickly to load round.
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