Technobeam - lamp strike issue

I'll preface this with the condition of the fixtures first. I bought them used, they all work great. None of them have displays or buttons (surprise) - I haven't been able to replace them yet. So all of the units are set for the lamps to strike when powered up.

Sometimes one or two of the lamps don't strike, and I have to unplug, and then plug them back in.

Anyone know why that would happen?
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