If your toilet flushes weak or does not flush at all, then check out the water level of the manufacturer. If the water level is below the manufacturer's guide or too above it, then it will cause malfunctioning of your toilet flush valve. Adjust the fill valve to the marked water line. Or half an inch below the waterline level. Suppose the flapper chain is not attached to the flusher. Then clip the chain with the flusher lever. If the flapper is closing right after you flush, then the chain has too much slack. Adjust the chain, cut the excess to have minimum slack. If your flapper is installed correctly but not working properly, you may have an incorrect flapper. Replace the flapper and have a stronger flush now.