Pumps do what they do unless there is a problem with them.
Pump impellers wear out.
Pumps sometimes have low voltage to them due to electrical issues.
Pump intakes can have air leaks and/or clogged foot valves.
The pump curve chart, along with the depth to the intake, would be the first things that I would look at.
Well, that and looking to see if some doofus installed all VAN nozzles.