I have a toilet that when I flush, the rubber stopper in the tank lifts to flush the bowl, but the rubber stopper will not fall to close the hole in the tank. It doesn't seem to be anything wrong or broken with the mechanics of the stopper. Could it be that the plastic that the rubber stopper is attached to is just stuck standing up because of what it's pivoting on (the plastic) is covered in grime?
By Loleini
There are "ears" with holes that attach to the flush mech. You can replace the flapper. The flapper is probably getting to flexible, so when it is up the part that attaches to the mechanism is too soft. Also, check your chain, sometime the chain position prevents the flapper from closing. But the best thing to do is just change out the flapper. Take the old one with you so you get one that isn't to small.