Stones are not well fixed in the piece

I got one of these and one of the stones fell out and was lost the second time I wore it. I sent it back and got a new one. I didn't wear it for a bit, but when I did, first time out same thing happened, a stone fell out and was lost. Once could be just bad luck. Twice with the same problem suggests a more fundamental issue. Very lovely piece, but poorly constructed.

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