Check that the flapper or stopper is centered in the flush-valve seat. Raising or twisting the flapper may solve the problem. If not, install a new one. Bring the old flapper to a home center so you can buy an exact replacement. Try a different type if the new flapper doesn't seal tightly.
Brush the rim openings with the mineral cleaner and allow several minutes for it to soak in; repeat several times. A mirror will can help you see if any openings are clogged. Auger a rim opening with a piece of insulated wire or a very small bottle brush.