I struggled with my Ecolink ECO-DW door sensor at first, until I finally nailed down the proper way to include it. The key is first to ensure it is totally unpaired, and then prevent battery from making contact again by inserting piece of plastic/paper in the slot.
At the location you plan to use it you will have to include it, which either means you have to run quick, or use a portable device.
It now works perfect for me, but you might as indicated by bkenobi simply have a defective unit.