There's a thermal shut-down feature. If they were very rapid, full/near-full power flashes then the unit may have overheated. You can always let it cool off for a while and then try again. It's also possible that the bulb isn't seated well.

If you want to troubleshoot you could swap flash bulbs between your Einsteins. Don't touch the tubes directly - use a clean cotton cloth to handle them. By swapping them you can tell if the problem is the bulb or the Einstein itself.