I have a SW ez Shield 9mm with grip safety. My problem is that the slide often stays open between firing. I've had gun instrutors look at it, watch me shoot and it still can happen.

You mean it locks back on a magazine with a live round or two still in the magazine?

WELCOME to the Forum!
Not enough specifics.
Do the gun instructors have this happen to them? Has anyone else
shot the gun?
Is your support hand thumb hitting the slide lock on recoil causing the slide
to lock open?
Have you shot all rounds in your magazine when this happens?
Can you give a few more details?

is it always the same mag that does this?
More questions than answers...