Flash Problems...

So I had a shoot this morning and my flash kept miss firing. It would only flash about 1 out of 4. I am just using a sync cable between my 40d and 580 ex II. I thought it could just be the cable but when I put the flash on the camera it was doing the same thing.... I changed the batteries, any ideas on what is going on?

Try cleaning the contacts of the camera and flash's hotshoes. Also make sure that the flash (or sync trigger) is tightened down on the camera's hot shoe. Other than that (and changing the batteries, which you tried), I'm out...