It can be a pain if your toilet won't flush the first time you push the handle. There a few reasons why this can happen, and they can be repaired.
1. Insufficient Fill
A toilet only flushes properly if the tank fills completely. Two things can affect tank fill. A lack of water, which can occur if the water supply tap isn't fully open, is the most likely cause. A damaged fill tube in the toilet is another possibility. Replacing the fill tube may be necessary.
2. Damaged Flapper
The flapper is the part that opens and closes to allow water to rush from the tank and into the bowl when you flush. If the flapper isn't closing and sealing completely after a flush cycle completes, then water will leak out and there won't be enough water pressure in the system to allow for a complete flush. Flappers can wear out over time, so they may require a replacement in order to work properly.
3. Broken Lift Chain
A nylon or metal chain is attached to the flapper. It is this chain that ensures that the flapper closes properly and at the right time. Failure to do so can result in weak flushes, especially if it closes the flapper prematurely. Some chains can be readjusted so that they close the flapper at the right time. If the chain is broken, though, it will need to be replaced.
4. Tank Leak
A leak in the toilet tank can result in too little water remaining in the system. Leaks can occur around the toilet handle or because of a crack in the bowl itself. If there is any moisture on the outside of the toilet bowl, then you need to inspect the tank for any signs of a leak. Another sign of a leak is if the toilet seems to randomly run without being flushed, as this means it is refilling after water loss from a leak.
5. Hard Water Deposits
Water must flow into the bowl unhindered if the toilet is to flush properly. If the emitters under the rim of the bowl are blocked by hard water deposits, then the obstruction may prevent the water to come in at the necessary pressure to force the bowl to flush and clear properly. A water softener is a long-term solution for hard water deposits, but in the short term, you may need to have the mineralization cleaned out of the emitters to improve toilet performance.
Contact a plumber in your area if your toilet isn't flushing properly.