5cm? Its time to "cheat"! When you stand and reach before you jump, don't reach so high!
During workouts, work your hamstrings with "hinge" movements like Romanian deads, pull thrus, stiff leg deads with dumbbells, etc. Focus on generating tension in your hamstrings, not just moving weights. When you bend over, make it "feel" like when you dip before a jump.
When you practice your vertical, make sure its practice. Jump smooth and perfect! Don't tire yourself out and jump around sloppy.