i had a suimilar problem. lift the lid to your w.c cistern if you have the old type ball valve, a common problem is a build up of scale deposits from the water. turn off your water supply or isolating valves if you have them, the ball valve can then be unscrewed and separated into its composite parts if you see any scale gently use a wire brush to remove this. then reassemble and turn on your water supply.
some more modern (bottom filling) float valves are fitted with a filter screen at the point where the water supply enters the cistern. sometimes these can become filled with debris which restricts the flow of water. it is sometimes necessary to remove the cistern to get reasonable access to service this tho.
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