If you jump, you pressed L/R too early or didn't press it at all. If you visibly leave the ground then airdodge back down rather than just sliding, you pressed L/R too late. See which one you do and adjust accordingly until you get it right.
Falco's wavedash timing is pretty slow (5 frame jumpsquat) so it's easy enough, and since you can wavedash with other characters, chances are you're going too fast. I suspect that is likely to be a problem people face when starting off since they might just be going as fast as they can for the characters with shorter jumpsquats, so they have to learn to actively slow down, which is kinda hard with a technique like a wavedash where you're pressing 2 buttons pretty much in one motion.
Jumpsquat durations
3 frames: Fox, Ice Climbers, Kirby, Pichu, Pikachu, Samus, Sheik
4 frames: Captain Falcon, Doctor Mario, Luigi, Mario, Marth, Mr Game & Watch, Ness, Young Link
5 frames: Donkey Kong, Falco, Jigglypuff, Mewtwo, Peach, Roy, Yoshi
6 frames: Ganon, Link, Zelda
8 frames: Bowser