It could be flooded and there isn't enough battery to clear it..maybe your motor needs more rpm of cranking power to get started after setting for a long time. I've seen and been in many different types of situations where a motor would crank but not start, so it was pushed and it started. Could have compression problems in several cylinders (especially a diesel). A diesel with low compression will crank like crazy all day and not start until you push it. I don't recommend push starting; it's dangerous and usually unnecessary, but if the advice of bypassing the neutral safety switch and cranking the motor while pushing can get somebody home where they can fix their car right, I'll be there with it.
