How did you try to release the slide?
If the empty magazine was in the gun, pulling back on its slide only makes the slide-lock dig in deeper. The follower of the empty magazine actuates the slide lock.
The only way to release the slide of a 1911, when it has an empty magazine in it, is to press the slide-lock lever down against the upward force of the magazine spring.
The only correct way to release the slide of a 1911 is to first remove the empty magazine.
Having first done that, then you can either press the slide-lock lever down, or you can "slingshot" the slide.
I am shocked—shocked—to learn that a gunshop salesperson didn't know that very elementary step.
It is true with many more guns than just the 1911.