A physical obstruction in the pump impeller is the most common cause of low flow. Often small items can get through the pump basket and get stuck in the pump impeller, they often cause other things to get stuck with them and soon there is a large mass of debris in the impeller that restricts the flow sharply. The solution is to access the impeller and remove the obstruction. If the pump basket is in poor condition allowing larger items through it can add to this problem so you will want to replace it.
Occasionally there are other obstructions such as something stuck in the plumbing between the pool and the pump, or even misaligned suction valves. If the skimmer basket is in poor condition it can make this condition much more common and you will want to replace it. If you have an automatic cleaner, you will want to check the cleaner and its hoses for obstructions. You will also want to check the pump basket, if it is too full, then the flow can be obstructed. This can be a frequent problem for people who have removed their skimmer basket to install a pool cleaner. The pump basket is usually too small to take the leaf load of an entire pool. We recommend the installation of a Vac-Mate or an inline leaf canister in this case (often both).
Broken/Worn Pump Impeller
Occasionally a pump impeller will physically break off of the motor shaft. Also a metal impeller can be eaten away by chemistry or age. If so the impeller will need to be replaced.