Engine Speed Problem

Hello. I’m trying to build a new f450 frame, pixhawk2.4.8 quad.

When I try to accelerate the motors there are only 3 speed levels.
Slow, Medium and Fast. There is no in between.
To explain in more detail, we gave power to the engines, they started to spin at idle, there is no change until we bring the speed bar to half, when the speed bar comes to half, the engines suddenly start spinning at medium speed, we constantly raise the speed bar still does not change, when the speed bar is at the top, it suddenly starts to spin at full throttle. Then we start to lower the speed bar, there is no change until it reaches medium speed, it continues to work at full throttle, when the bar comes to the middle, it suddenly drops to medium speed, then it comes to medium speed. at the bottom, it suddenly drops to idle speed, so I can only get 3 levels of speed.
The Remote Speed bar normally increments millimeter by millimeter in Radio Calibration in the Mission Planer program. But the motors only give me 3 speeds. What should I do so that the speed gradually increases and decreases gradually according to my bar movement? ,

Please Volume Up and Watch: