You have to charge the jump meter with the mouse in casual mode. Check the description for more in depth instructions and if it still doesn't work, I'll check it out.
By the way, here are some pointers to help with the timing and jumping in general in normal mode.
When you're trying to bounce, you usually need to get the most out of your jump, so it's usually best to press and hold the jump button again immediately after you jump. If you do that, even the shortest (as in least high) jump you can perform gives you enough time to fully charge the next jump.
As for the timing, if you've noticed, when you hit the ground there's a small landing animation. As long as this animation is playing the jump meter does not reset. That's when you must release the jump button to get the timing right.