There are three general possible causes:
1. The slide "release" fails to see lift from the empty magazine follower. This could be a magazine issue or a "release" issue, if the tab on the "release" is not within spec.
2. The slide "release" is binding. Check by field stripping the pistol. With the slide off, you should be able to lift the "release" with a wooden toothpick and when you let go, it should drop of its own accord. If not, check for a burr in the well in which the "release" resides, or possibly an oversize pin or undersize hole. If you pass this test, reassemble the slide without the recoil spring, no magazine, hold pistol upside down, and draw slide back. The "release" should fall into place. If not, check for a burr or other obstruction in the relief into which the "release" is designed to fit.
3. The slide is not traveling back far enough for the lip of the relief to clear the "release." This is most likely an ammo issue. This is also the least likely cause, particularly if the pistol is otherwise ejecting and self-loading per design.