This happens a lot in Konquest. You have to make sure that you did no other previous combos to Sub-Zero. Basically, if Sub-Zero is bleeding or like holding his face or anything, wait until he's standing normal then perform the combo, if you don't, the comp recognizes it as a chain combo or something I guess. And another thing, some of the combos are EXTREMELY precise in timing. Just mess around with the timing of when you hit the buttons and you should be able to do it.