I see a problem in your last tutorial. when you press the left arrow key and the speed up key
("X key") to make the player go faster the player can't jump. But It works fine when press the right arrow key and the speed up key ("X key").
I can do this no problem, it will probably be your keyboard (not something you can fix)
Try holding left & X, then pressing Z in notepad, see if Z comes up.
I can't remember what it's called [Edit*: Yes I can it's called ghosting] but there's something that some keyboards have that makes them not able to press some keys if other keys are down