possible fake diamond from Sam’s Club

14 months ago, we had some money from insurance claim to replace two stolen rings. My wife bought a trillion cut ring ($1,500) from Sam’s Club to replace the second ring. Over the last few months, we have noticed that the trillion cut ring no longer shines. Two jewelers examined it and said it had been coated to make it shine and the coating lasts only about a year. Another jeweler said he did not believe it was a real diamond. The sales person at Sam’s was new to the jewelry department and said she could not find any papers on the ring when we bought it. She was suppose to get with the person who usually worked with jewelry and mail the papers to us. They never did and we forgot about it. We do have the sales label identifying the ring as a real diamond and are now contacting Sam’s Club about the problem.
Have you heard of similar problems at Sam’s before? Any suggestions on how to proceed with this?