I'm wondering if it could be a browser/OS issue? (Unity WebGL is not well supported) It sounds like you're doing it right but just for sanity I'm writing out the process below.
Jump: You must be touching the ground in order to jump. When the ground is flat, you can press space to make a small jump, but the jumping turns into bouncing as the trampoline starts to stretch downward.
After gaining a little momentum, jumping works like this:
1. wait until you're in contact with the trampoline
2. hold down space
3. continue to hold space until you are in the air again
Fall Faster: you get extra downward force as long as you hold down shift. The force is scaled with how high you are when you start pressing. It's applied continually until you touch the ground, then it resets to 0.
Let me know what browser/OS you're using and any updates if you try again. Thanks for bringing this to my attention.