What expression pedal is it? And would you happen to know the resistance of the potentiometer inside? 10K to 25K pots tend to work best. Also, is it a linear or logarithmic tapered pot? That can affect things as well. Linear is best for expression pedals, logarithmic is more suited to volume pedals.
And unfortunately, not all expression pedals are built the same.
2, 3, 5, 13, 89, 233, 1597, 28657, 514229, 433494437, 2971215073