My zwave sensor is doing the same. There also appears to be a short delay before it is able to pick up motion once it has deactivated. Vera were helpful and got one of the sensors working fine. I emailed smartthings who got the same sensor working on the ST (once unpaired), however it stopped working with ST the next day.
Some of their other products worked better (smoke detector, water sensor, plug and door). I have been tempted to get some of the zigbee things as they are much cheaper.