angelia> usually diamonds will drop out or bcame loose, due to the "claws" that r holding them. sometimes without us reaslising it, we hit against some things, causing it to be unstable.
what i would suggest, once in a while, use the tip of ur fingernail to "knock" onto the diamond, to c if it's shaking. (before it really came off the setting.)
actually, to share with u ladies something (which most sales person won't tell u). try not to send to the outlets for "washing" too often. cos they use machine to "shake" off the dirt. doing that too often will "shake" the setting oso. So to maintain the shine of ur diamond, use a used toothbrush (of cos not too super dirty one lah) plus some mild detergent (like mama lemon), hold the ring upside down, & brush the bottom of the diamonsd (the gap of the "claws"), after which then the surface of the diamond. use a dry towel to dry it (dun use tissue or toilet paper and try to use those towels with less fibres).
HTH