You may have to open the tap and have a look at inside.
Off the supply to the basin.
You need a scewdriver, spanner and some oil.
Remove the cover on the handle of the tap.
Then remove the handle up and away.
You will then need to a spanner to remove the
next part out of the tap housing.
Have a look inside is there any sediments built on the moving parts. If the rubber washer is worn you many need to remove it. The rubber washer should be fixed to the mechanism and move easily up or down. Oil if necessary.
Fix back the removed parts in the reverse order.
Good luck
ma, March 2009