could be as simple as bad contacts on the lamps.
Or, could be bad design/LED's.
I had chinese ones that got dimmer and dimmer, and a few even unsoldered themselves when the converter was on because of excessive heating. But not ones that flikered. (Well at least not till they gave one last flicker and turned into a DED (darkness emitting diode)
Inexpensive ones often have only a resistor to set current, not a current regulator. Excess current kills LED lights in short order. to low a current kills light output drastically. Big balancing act for a device that is by it's design highly nonlinear for current vs. voltage, and highly temperature dependent.
High quality ones have a current regulator to hold constant current to the LED regardless of LED temperature and incoming voltage. But then those aren't inexpensive either.
Options are to go with high quality ones with a regulator and pay the $$$ or find a decent chinese version that works.
Cheap ones may work well forever, or may like my first try, suffer from overcurrent and end up gradually getting dimmer and dimmer till they are a nightlight.
but ones with good regulators that don't cause RF interference with radios, tv's etc. aren't inxpensive, figure about $10+ each. But they are BRIGHT. mine are noticeably brighter than any of the imports I've tried. And light output and current drain (power) is constant from about 11V all the way out through 18 or 24 volts depending on the voltage range they are designed for.
superbrightleds.com is one source of high quality regulated ones, but again not inexpensive either.
Pays your money and takes your chances.
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