Normally the FrameKit sits stably on the iPhone screen. Like all industrially manufactured products are, however, both the iPhone and the Eassee3D FrameKit have minimal manufacturing tolerances in 1/10mm area, which can cause in exceptional cases actually that a FrameKit sits slightly tighter or looser.
A gentle pressure on both sides of the frame before being clipped usually corrects the problem. But please be gentle!