Well, for starters, who do you play against usually? that probably would help ppl help you.
Assuming that you play against human players, the first thing you want to do is know everything about Fox and his moves. You can do this by reading the various guides to Fox on Smashboards and by simply practicing. This is important so you can decide quickly which attacks you should follow up with in any particular situation.
If you've got Fox's moveset down pat, and you can't consistently combo your friends, then you should try to work on your mental game, as this may be a factor in your falling apart. Try to read your opponents, and not just situationally either. If you usually play with the same group of ppl, then you should be able to pick out various tendencies that they have, such as which way they usually tech (most ppl have a preferred course of action even if they don't realize it) or the way they tend to DI or how they attempt to interrupt your combos.
Knowing things like this will allow you think more clearly in battle, and you'll find that you'll be much more successful in predicting what your opponent is going to do if you just take the time to think about it. A good mindgame combined with a good knowledge of your characters moveset will allow you to take a situation (even in the heat of a combo), analyze it, and act accordingly.
Lastly, something you should consider, is watching some vids of pro playing. Doing this will give you tons of ideas for combos that are actually practical in battle.
Hope this helps. definitely read the fox guides if you havent already.
