That's a very odd question, considering all of those hits gatling, and they gatling very easily. All I can say is make sure that your timing is correct, watch videos to ensure that you have each rythm correct. The other possibility is that you actually mean block string, instead of combo?
Also, if you feel like you hit a glass ceiling, make sure you're adding in tech traps, airunblockable traps, overhead/low mixup, command grabs in pressure and other tricks.