Changing motor direction using K8055D

I’m trying to control motor direction using K8055D in Matlab, I’ve connected the motor to the digital outputs using H-bridge.
However, I want the motor direction to change once the value of external pot that is connected to the analog input changes.

Here is the code I worte:

if (calllib('K8055D','ReadAnalogChannel',1)==15) calllib('K8055D','SetDigitalChannel',1) pause(1) calllib('K8055D','SetDigitalChannel',2) calllib('K8055D','ClearDigitalChannel',1) end

It works only if the pot was set to 15 before I run the code, otherwise it doesn’t work.
I’ve tried to put the code in a loop but it didn’t work.

I need help please …