This could be due to a split non-return valve, located at the base of the pump mechanism or perished washers and seals. A replacement seal kit is available from Consumer Services, please email email@example.com.
It’s important to distinguish between losing pressure and not achieving pressure. First, remove the pump and place the bottom of the pump against the flat palm of the hand (wearing rubber gloves if a chemical is present). Pumping the pump should result in pressure being felt coming out of the black non return valve of the pump. If this is happens, then the bottle is probably getting pressure but subsequently losing it. If no pressure is felt then the sprayer isn’t getting pressure in the first place, so the most probable cause is a faulty, missing or dry plunger O-ring or leaking non return valve. Replace or lubricate as necessary.