I can think of 3 possibilities-
1) If you are connecting another controller to the 7090 you may need to calibrate the minimum speed as their was a small change in the signal voltage. This only applies if another controller is used and the 7090 is run in external mode.
2) The plug to the pump inside the controller may be loose or have a bad connection, try opening the controller and checking the plug, unplug it and plug it back in.
3) The propeller assembly fits too tight on the shaft and stalls at low speed. If you have two pumps you can confirm this by swapping prop assemblies, if it follows the prop, that is definitely the cause and we can send the prop assembly.