Radiator fans wont speed up when cpu gets hot

Ethanh100

Honorable
I have a cpu water cooling loop in my system and i have a problem where the fans wont speed up according to temperature. When in the bios, i can manually set them to whatever speed i want and that works but the fan curve will not bring the fans above idle. the fans are 3 pin, which are connected to my phanteks fan splitter which is connected to the cpu header on my mobo by a 4 pin cable. Why is it doing this and how can i make it works? thanks!