What moisture sensor are you using? That's always been a sticking point for me, where soil moisture sensors just don't work, unlike air moisture sensors.
Due to the volume of the pot, it would be difficult to get a true moisture reading, so I place the sensor directly under the spout in order to have noticeable changes.
http://www.mywildeye.com/wildeye/soil-moisture-online/
which can use a number of different sensor makes, but http://www.decagon.com/en/soils/volumetric-water-content-sen... seem pretty good