The problem could be due to alignment of the lever and hook which pulls the pumps up. Make sure it has a free play.
If your toilet a siphon type. You probably need to take a look in side the cistern. Remove the cover, flush and take a look inside. Ensure there is no blockage on the moving parts inside the cistern.
Clean all the joints, if there is any deposits of lime on the handle joints remove it. Oil the joints.
If that does not help you will have to take a look at the suction, there could be a spring and weight to restore the position of the lever back to normal position.
@ma
December 2008