The only force that sends the magazine out is from the magazine spring right? Does an empty magazine go further out when the slide is closed as opposed to locked open? Mine does as the slide catch puts more force on the follower when closed.
I'd look close at the slide catch as well as the magazine spring...and don't forget the mag catch. I've found that if I hold the magazine somewhat in while releasing, and release the mag catch button, the friction caused by the mag catch on the body of the magazine will stop it in its tracks.
All of these statements apply to an empty magazine.
Here's a test you can do:
Insert an empty 7rd magazine and lock the slide open. Then point the muzzle upwards and try to make the handle (grip) perfectly level. Then, while only only holding the slide, press and hold the mag release as hard as you can and keep the button in all the way. The magazine should come out as far as the #7 round inspection hole of the magazine. If not, try angling it down a tad bit and do it again. All it takes is a slight decline in angle and the magazine will damn near slide out for me.