shower mixer taps. cold tap hardly works anymore

Hi,
I have a basic shower mixing tap in my shower/bath and over the past few months I've been able to turn the cold tap on less and less (if you know what I mean?). The hot tap seems to be fine. It's now gotten to the stage where very little cold water is getting through ans the water is almost too hot for a shower.
I've tried unscrewing the cold tap but there's no budging it. Has anyone out there experienced this before. The taps are about 6 years old. There's no problems with any other taps in the property.
THanks in advance,

Turn off the cold water supply to the tap in question and remove the tap head from the mixer body. You can then take the "guts" apart(tap components)) and grease them up(Vaseline). This a very common problem on cold water taps, especially in bathroom areas. You will need a screwdriver and an adjustable wrench to do the job. The screwdriver to take out the screw that holds the tap head onto the tap body and the adjustable wrench to take the tap body out of the bath mixer. reverse this procedure and you should be ok