Toilet cistern problem?

Hi,

my issue appears to be that the volume of water is slightly less than is required. I can see the fill line but water fills to about 5mm below the line. The toilet doesnt respond to a normal flush, it will eventually flush if the handle is pushed severely several times. If I press down on teh ballcock and allow the tank to fill to the level required or past it the toilet flushes okay. Any hints/advice much appreciated