Why do my cutters not work?

https://preview.redd.it/k4s8jkvkh7md1.png?width=831&format=png&auto=webp&s=2b8c43b8e5f38c13ea7701d22ed0b2db4eff8f21

Can somebody tell me why my system fills up? On the right there are 120 circles entering the system. I have 4 cutters, every thing is on the same level. Even the game tells me that 4 cutters can handle a 120 belt. But my system still fills and I only get about 80 at the end? Am I making a mistake?