It actually does fill, but you also loose combo at the same time so you have to be quick with your fingers. Its also very important to do it correctly and release the key before pressing the next one. With DNA you can make it fill faster and also make it so you loose the combo slower. The higher combo you have the faster you loose combo aswell. Thats why DNA upgrades in the ascension are so important if you wish to go for combo.
I made it like this on purpose so you dont get 3x or 4x early game but with one upgrade or two you will get to 3x without any real problem. Late game would be more suitable for 4x.