Is the skimmer configured for "in-tank" or "in-sump"?
If configured for in-tank mode, the skimmer must be mounted with the water level at the "blue-line." Otherwise, the water level inside the skimmer will be too low. The bottom plate used for in-tank mode has an open grate.
If configured for in-sump mode, the solid plate on the bottom with the suction cups must be sitting on a firm surface to form a seal. Using the riser tube, the pump will raise the internal water level to the "blue line." Without a platform or the bottom of the sump to rest on, the water inside the skimmer will leak out and fail to reach the "blue line."