If you look at the bottom of your screen where the screws attach there's two screw holders attached to the screen. These can sometimes get bent, misaligned or even break off completely, if they're bent or misaligned you can realign them by force, if they're off completely there's nothing short you can do of either supergluing them back or simply adding a light adhesive to the screws themselves.
This sounds like you have used the screws that go there somewhere else on the phone. This could cause damage. I would go back over the repair you made and recheck that you don't have those longer screws in the wrong holes.
