Diamonds fall out withing 2 days even after repair

I bought my Fiance a ring in August 2015. I left in in the closet untouched till 16 December 2015 when I proposed to her. 2 days after that we noticed one of the diamond fell out. I calmly went to Sterns to get it repaired, no problem. Received the ring back within a week which was quicker than expected. Not even 3 days after she had the ring back the same diamond fell out. What kind of cheap craftsmanship is this. For R4999 ring you would expect a bit more attention, especially now that it messed up everything for her, as she don't want the ring anymore and will be too scared to ware it or trust anything that comes from Sterns.