When your toilet takes several flushes to clear, the first thing you should check is if the water in the tank is low. Adjust your float to increase the water level and ensure that the level is at least an inch below the overflow tube.
It could be that the flapper closes too soon. Check the length of the chain. Adjust the chain slack to about a quarter to half an inch of slack.
Check the fill tube between the fill valve and the overflow tube. The first flush will serve merely to fill the bowl while getting ready for the next flush, if the bowl is not filled prior to flushing.
Finally, it could be that there is some material that is clogging the trap-way of the bowl, the trap, vent pipe or drain pipe and slowing the siphon action of the flush. Use a plunger, a toilet auger, a snake, or a blow-bag to unclog.