If the handle in the back is not going completely down, check to make sure you are using the correct cylinder. The cylinders with the clear seal and blue label will not work with your ART Sparkling Water Maker. 

To properly insert the cylinder, first raise the pink cylinder handle. Remove the seal and cap from the cylinder and position the bottom of the cylinder in the Sparkling Water Maker. Then gently place the rest of the cylinder into the cylinder holder. Lower the cylinder handle to lock it and replace the back cover so it clicks into place.