The toilet is fickle?

our main toilet is fickle. when we flush the toilet...the little stopper does not always go down to cut off water. my mother with dementia has the toilet running all night with the water running..lets just say.our water bill is twice as much cuz of this little toilet. i took the tank top off and everything looks good. i hate to have to spend hundreds of dollars for a two minute repair..i am a dolt at repairs..is there something i can put on the hinge area of the stopper?. i cleaned the area but as i said the devise is fickle..is their some 'gunk' to place around this hinge thingy to make the devise smooth running and not so fickle in operation..this is not rocket science.lol.advise appreciated.