Learn some simple repairs to fix a weak-flushing toilet in this video.
If the toilet won't completely flush or if you have to hold the lever down, you may have a problem. Sometimes it won't completely flush because the flapper is not getting enough lift to drain the tank.
Be sure to check that the chain is attached to the flush lever and has enough give and pull. Check the chain for slack. If the tank is not getting enough water you may need to adjust the fill valve.
Depending on the valve type, you can adjust the water level by either rotating the fill valve body, or by controlling the float height with a screw adjustment.