Hi,
I’m not sure if this is what you want but you could remove the touching down check inside the if() statement as that way they will always be able to jump once before having to land to reset the jump counter. The drawback to this though is if they for example fall off the platform they will still be able to make a jump as they are falling down. If this is not a problem it could be an easy solution.
Sorry if this is not much help 