The documentation says to double-click the jump button
Actually, you need to hit the jump button and then
double-click quickly--three taps on the jump button in
all--before your character hits the ground.
When in doubt, try a quick pan with your camera-view controls.
A new perspective on your situation can often be enlightening.
Unless you have a gamepad, which is probably the best
alternative, I recommend joystick controls.
It gives a finer touch on the controls than the mouse
or keyboard, and you can configure the buttons to suit