Then it's bad that way, you can't let the player spam combos, you can chain multiple hits of course and the rival must eat each hit (except if you have a combo break mechanic), but if you fail hit, or the rivals guards your combo, in the last punch, you must be wide open during a period of time lesser than hitstun of the enemy in block, so he can punish you. If you land the first hit there is aframe of time where you are able to connect the next move of the combo, or chain a special or something, but if you dont make the next move inside that time (in casual games like this, it's the time is big so you dont need exact timing or good execution, which I recomend for this game), you drop the combo, and the rival can punish you, or fall to the ground (this depends on the intesity of the attack you throw before drop the combo). As I explained before, you could create a simple combo system, that limits things like this, or limit the times you can be hit in the air.
It a deep topic which I uknown, maybe an experienced fighting game player can give some tips. You can base in other games. I learned a bit of this because I watched a lot of fighting games, and also made a char in mugen.
But in games like rumble arena, I think the combo are simple, 3 or 4 hits and the a special or to, maybe a special that throws the opponent into the air and can continue the combo in the air with 1 or 2 hits, like I do with Wargreymon, some normal hits then front tornado + up tornado for example. I think the system you have is fun but need polish, but i wouldn't call a bad system or wrong.
Sorry I can't help you more