Diamond fell out

I have purchased my wife wedding ring just over 4 years ago December 2014 frome nwj tygervally mall and got it resized before taking it out the store. Today we noticed that a diamond has fallen out. Why would a ring purchase from a professional jeweler fall out? Very very upsetting and disappointing. I have the similar problem with naranda jewelers in durban and had issues with the repairs . Please tell me my way forward that does not bare cost to me before I turn to customer protection services and legal Wise