First off, you need to determine whether the water flow is adequate at the suction connection. Do this by using your little flowmeter that came with the cleaner. Disconnect a section of the hose while the cleaner is running and place the flowmeter in the end of the hose pulling suction. Ensure that the flow of water pulls the flapper between the two lines on the flowmeter. If it doesn't, adjust your cleaner line valve at the pump until you have the right flow.
If your flow is fine and the cleaner still isn't running with gusto, you can suspect a worn out diaphragm, foot pad, or both.
The diaphragm is the rubbery valve thing inside the cleaner. It is white and has a "mouth" that opens and closes to create the clicking sound you hear when it is working. After exposure to water and chemicals for a while, it hardens up and won't work as well. These are cheap and easy to replace.
The foot pad is the little yellow or blue shoe on the bottom. The fin disc which is the rubbery disc that is about 15" wide or so, fits over the foot pad. Anyway, the foot pad is subject to wear much like a tire does. It skids along the pool surfaces, wearing down all the while. It is recommended that you replace it when the tread depth gets down to 1/8". Also a cheap part and easy to fix. For what it is worth, the fin disc itself will wear out over time as well.
Check these things out and see what you find out.
