Our Stompboxes will work with any Expression Pedal that is a simple resistive load (potentiometer) with a max value of 20kOhms. We've successfully used the Roland EV-5 (set the trim pot to 0), the Line6, and the Boss FV-500L (connect with a stereo cable).
If your Expression Pedal has a trim pot control, try adjusting it to either end of its range.
Sometimes pedals pots may be not accurate or dirty. Some spray cleaner for electrical contacts can be applied to clean the pot.
all the best