One of the most common toilet problems not flushing right is having to hold the flush lever down continuously to flush the toilet. Fortunately this is an easy fix.
The problem is caused by too much slack in the lifting chain which attaches the flush lever to the flapper ball. When the chain has too much slack, it cannot lift the flapper ball high enough to let it float on top of the water in the tank and it closes prematurely, thereby stopping the flush.
To fix this problem, simply adjust the chain length so there is about 1/2 inch of slack. If the leftover chain hangs down too far and interferes with any part, trim the leftover chain length.