Unknownlight
Member
This happens frequently enough that's it's been bothering me, but it was also rare enough that it took me a bit to realize what was happening.
When you run up a slope, there's a brief moment during the transition from sloped ground to flat ground that your character isn't actually touching the floor. If you press the jump button during that time it doesn't work. You're "in the air", after all.
The best way to fix this would be to implement "Coyote Time" (where you're still allowed to jump for a few frames after leaving the ground). Most platformers do this, and it's a good QOL improvement in general.