Unfortunately this is becoming more common.  The best way to protect yourself against diamonds falling out is to ask before you purchase, specifically about what guarantee they offer in case a diamond falls out.  Ask about what happens if the diamond falls out in the first year and subsequence years and get it in writing. If they are unable to offer a guarantee in writing or other details then they may have concerns around:

• Inferior quality metal casting (pouring into a mould) so the metal is defective.
• Using a money saving technique where the diamonds are cast into the ring and then they try to tighten them.
• Cost cutting so that there is inadequate metal to hold the diamonds.
• Poor design as the designer doesn’t understand the engineering of diamond & gemstone setting.
• Poor workmanship caused by inadequate training.
• Paying stone setters per stone causing the setter to rush in order to make an adequate income.