Poor Workmenship

I firstly bought a ring from American Swiss at Sanlam Centre about 3 months ago, the ring had to be resized and i got it back...about a week later the ring started turning black and u could clearly see the lines where the ring was made bigger. I took it back and it was found that a different type of Material(Silver) was used to make the ring bigger. After about 1 months struggle i managed to get a full refund. 3 Weeks ago i decided to try another American Swiss at N1 City, i made the whole issue clear to the staff on the floor. They confirmed it would not be the same case, and guess what. I bought a new ring there, i had it made bigger and came back with a very rude Manager phoning me to come collect the ring. I had it on for nearly 2 weeks and exactly the same problem. Now i am being told that i am allergic to the silver of which i have about 9 other rings i have worn almost my whole live with not 1 going black or showing the marks where it was made bigger. I am absolutely gutted by this response as i had the ring valued by a professional jeweler and i was told that the material being used on this specific ring was more that 70% Copper!!! How can we as clients spending so much money on Jewelry by companies such as American Swiss which i trusted my whole life because of the brand name, is doing this to its clients. Absolutely unacceptable!