The inconsistent performance of controls can only be placebo as all of the buttons are mapped to the same input.
Your timings must be very off that the inputs don't register correctly when you do springjumps and longjumps. You're supposed to be pressing shift and space roughly at the same time. I implemented a double buffer system where you can be both early and late on the timing and it should still count.
I'm currently working on an update that aims to improve the tutorial around these mechanics, and makes the game slightly easier overall.
Thank you for your detailed feedback.