2 out of 4 flashes stopped working

[sandralovesyou]

I have three 600 EX flashes, and one 600 EX II flash. After not using them for 2-3 months I pulled them out for a gig and two of them seem dead, and the other two are working but one is intermittent. I cleaned the contacts and tried new batteries. It seems so strange. Any suggestions for why they would randomly die like that? Can I revive them? Thanks. 

[kvbarkley]

What do you mean by "dead"?

No response at all?

Unit turns on but does not flash?

Do they work on-camera?

[rs-eos]

Were the batteries left in during the time of non-use?  Did the batteries leak or cause any kind of corrosion or other damage?

Also agree to the question about what "dead" means.  Note that when batteries are moved (or holds no charge) for a period of time, any settings in the flash will get reset (e.g. your unique ID value).

If they do turn on, what happens when you press the test flash button? Does at least that work?