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Have the dimmers been approved for use on that make sort of light, or triid out in advance ? At the very least are the dimmers set to be trailing edge cut, rather than leading edge cut as you would for a filament lamp.
edit. The Vtech dimmers (and some others) can be configured for either type of dimming with flicking on and off to get into the programming mode - has someone been playing with the settings?
It possibly worked fine when you were there since the loose connection was shiny and new, it is now a bit oxidised/stressed.
Or possibly they need the missing trims to work properly
Edit; Does your client know that a video of their bedroom is now on the Web for all to view?
But as you never intended it in this case then yes it works a treat, Murphy`s Law a.k.a Sod`s Law.
Yes I`m thinking of poor connection too. Poor switch or grit in switch not impossible?
(rusty old Fords with more body filler than steel were prone to this...) and wondering if there is a continuity neutral problem.
Switch mode LED driver modules are often a "negative-resitance" load - that is to give constant light over a wide voltage range, the electronics is arranged to draw more current at lower voltages to compensate, and with that type, if somehow you put 2 in series the system is unstable.
Unless perfectly balanced with equal voltage and current,, the one with less voltage will try to draw more current, so lowering its equivalent resistance, so lowering the fraction of the total voltage across it repeating the spiral, until it goes out altogether and restarts.
Could it be that ?
But I cannot see how you would end up with 2 in series across the mains without a wiring fault of some kind, and I struggle to see how that could occur after you left. Unless it was just balanced by luck.