I don't think I've ever experienced this while playing offline, but online, it's a different story. It seems like whenever I hit the c-stick at the right time, my move comes early, but when I do it in advance, the move occurs on time. Occasionally (though it seems to be mostly lag-induced), it won't work altogether.
Similar stuff happens with my shields while playing online. I'm usually good at blocking projectiles (I have to be, I main Marth), but I always get hit by them online, even if I know I'm inputting the R/L at the right time. Air dodging shows similar problems.
I dunno. I get pretty bummed out when I read threads like this, how the game seems to purposefully make it harder on Melee veterans (tripping, GC controller issues). I'm not one of those OMG liEK BrAWL SUX NO WaVeDASHING people, but this is none too amusing.