First, make sure that the shutoff is fully opened. If that doesn't solve it, if it's a multi-turn valve verses a 1/4-turn one, I'd suggest just replacing it. It's possible the fill valve has some crud caught in it. Depending on which one is in your toilet, if it is the one made for them by Korky, you can replace the seal for about $3 (buy it at Lowes or a plumbing supply house). Takes about a minute, instructions on the card. If your fill valve has a screw on the top, if it is the bad part, you need to replace the whole valve. I'd vote for the shutoff, though.