Break the combo up into smaller pieces and do those pieces first.
In other words, if the combo is B > C > Hell's Fang, then do the B > C part until you can do it without messing up, and then add try doing it again but with Hell's Fang tacked onto the end. If the combo keeps going then just keep going in that manner until you can do it all. Then practice it several times in a row.
Just because you did it once after trying 50 times doesn't mean you can do the combo. You have to be able to do it consistently, so try to do it 10 times successfully in a row. It will take a while though.
But before you even try to do combos, practice doing your separate special moves consistently.