If battery is low, it will turn over, but not fast enough to start.
Short list:
- check spark,
- check plugs,
- make sure fuel is sucked through. Pull choke on, kick over for 5-10 seconds. Then press throttle a few times listening for a squirt noise. Repeat 6-12 times until you here fuel. If takes too long, will drain the battery too much.
- Check battery power.