That's a very open ended question, there are so many possibilities of why it might not start consistently?
How exactly did you check the battery?
You need to put a meter on the terminals and let us know the readings with engine off, ticking over, and slightly revving, to try and see what condition the battery might be in? (when it does start obviously)
Make sure you have the headlights switched off when trying to start the engine, because they will zap your battery for power if it's not in good working condition.
Sounds daft, but I would lift the tank and firstly make sure the fuel tap is switched fully on, and then check that the fuel pipe coming from the tank isn't kinked in any way restricting fuel flow, especially when you rest the tank back down again.
New set of spark plugs never hurts, especially if you don't know the history of the bike, and a similar case can be made for the air filter (and fuel filter).
When trying to start the bike, hold the clutch in, to separate the clutch plates. If this helps then maybe an oil change wouldn't go amiss either.