I have 2 400ex's and neither them will start.
The one wont turn over. I tried push starting it and it worked once and tried it several times after that and doesnt work any more. Any ideas what is wrong with it???

The other one will turn over but not start. Could this be caused by a loose spark plug.

 

The 400ex will need a new spark plug, once in a while. I would try 2 new plugs and one new battery. That will probably get them going, if they ran when parked.