We have a jet pump which lifts water from the open well to 2nd floor tank,nowadays it lifts only 1st floor?

Question:We have a 1 hp Jet pump (Brand name -"Best") for lifting water from the open well to overhead tank located at 2nd floor,nowadays it is not lifting water properly,the pressure is very very less when it(water) reaches 2nd floor whereas it is somewhat good at 1st floor.I have tried the pressure adjusting bolt and nuts but in vain.can any one tell me the reason for the sudden decrease in pressure ?(the water level in the well is almost equal to the ground level and the depth of the well is 30 feet also there is no need to priming as the water stags on the pipe always))

Answers:
check the intake for blockage
check your pump performance against the manufactures publication (see link)
Is there a strainer on the intake? may be a broken impeller