Excellent ideas here; thank you!
Technical Support wrote:
A couple of things.
1- have you replaced the battery, ever? CyberSyncs generally don't trigger late, but it could cause a failure to fire if the battery is low.
I have not, no. I figured that with just a few thousand fires and less than one year old there'd be plenty of juice in there, but I can certainly try that.
But note that the other flashes always fire even when the one "suspect" one doesn't.
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2-Have you checked the EXIF and ensured the shutter speed is indeed 1/125 or below? I have had cameras whose shutter speed dial will roll inadvertently (usually hanging from a strap) and I have seen where settings change for no apparent reason in some cameras. Conversely, Pentax DSLRs dont trigger at all above x-sync speed (or that used to be the case).
That's right, Pentaxen never fire above the x-sync speed. And I've been using my cameras in the X position lately so the shutter speed is locked at 1/125th (or 1/180th for my K-3) and is not adjustable even by accident. The camera mode dial has a locking button so it can't be bumped.
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3-Do you shoot in the same environment every time (or everytime there is a failure)? Perhaps try changing the frequency.
I will try that.
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4-Have you tried shooting at 1 click slower than x-sync? Are you in rear curtain sync? Are there any flash settings which may be unintended?
I tried 1/60th sec for a while, yes.
Definitely not in rear-curtain.
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5-Can you try the PocketWizards for triggering in lieu of the CSXCV remotes and see if the lights still fire late.
That's a great suggestion, thanks.