Inspection by another jeweller is a good idea. Stones of any kind
which are set by a person with professional training should not fall
out of settings. At worst, they may become loose over time, but not
leave the settings.
In my experience, it is seldom a materials issue. I have repaired
other people's settings for a generation, and it has become
strikingly obvious that there are some wonderful craftsmen and women
out there who are self-taught in setting. Proper technique is rare.
If stone breakage due to rough wear were really the culprit, the
damage would be evident on the remaining stones as well. It doesn't
sound like this is the case.
A loupe reveals accuracy in bearing-cutting. Thickly-girdled stones,
such as round emeralds or sapphires with bulbous pavillions, require
careful attention to the depth and height of the bearing, and
appropriate thickness of the material used. A good setter should be
able to create an accurate bearing in ANY material, including steel,
with minimal risk of breakage.
I'm so sorry that this has happened to you. Dissapointment in a
custom piece doesn't sell much more jewellery down the road, nor
does it lend credibility to any jewellers, period.
Take the time to find an expert, and have the problem sorted out, so
that you may wear the piece and enjoy it without fear.