As mentioned by others, I think you need to disassemble the pump and do a thorough cleaning. Take the impeller assembly out and clean both the impeller and the pump internals with clean water and a brush. Also, the venturi needs to be completely free of any calcium deposits. I use a drillbit for this purpose. Handle the parts with care as the parts are fragile.
If you are using the neck extension, remove it. I always set the outlet flow (the two knobs) so that the arrow/indicator is perpendicular to flow. This seems to maximize the water level in the chamber, although the printed instructions are not entirely clear on this.