Circuits are very unreliable although looks ok from outside

I bought 3 sets. Out of box, 4 of them did work. After 2 days, another 3 broke. I am an electric engineer and fairly handy. However, they build the candle in way that is not for fixing. Also, there are a few candle just turn on and off randomly. I guess it is also caused by unreliable circuit inside.

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