Seems like others have given the probable cause and fix. If not working with adjustment, just replace it. Not sure your locale, but my local hardware store sells a base model for $12. Or Amazon/online. Float valve is the name I believe. Easy replacement - turn off, and the disconnect water line at toilet after you flush to empty reservoir, unscrew the whole valve from underside of res (good idea to have towel/bucket for small residual water), replace and screw in new one and reattach water line. Shouldnt be more than 20mins if never done it before. Any other questions should be easy to address here. Good luck!