When doing a true combo in training mode a block means the move was late, and if the move doesn’t come out at all, it was early.
The timing is probably more strict than you imagine. I know its a simple answer, but go into training mode and put in some time practicing the combos until you get them about 8/10 times. Then you can move that combo into field testing in endless mode to see if you can pull it of under pressure.
Offline timing is much more precise, but if you don’t play locally, that doesn’t matter for you.
Hit up the character specific forum and ask for some execution tips.
Watch some videos
Turn on inputs, sometimes this can help clear up what exactly you are doing and can iron out and odd execution tendencies.
Hit fewer buttons. Hit only the buttons you absolutely need to!