I find that most times combos don't come out is due to lag (netcode) as most mentioned.
Trying to connect Kotal's F2 in a combo with bad connection or even mediocre connection is almost impossible.
What I find what works best online is to try to get a feel for the "lag" or input delay on certain strings and specials and adjust.
Its easier said then done, of course, but I find once you get used to it, you can almost play in the worst of connections.
I get asked a lot in KOTHs and even 1v1s, "How are you SO good?" and I simply reply, just a matter of practice and timing.
Idk, its just my 2 cents.
Carry on.