Ok, this is similar to my experience. I've bought 3 pairs of eyeglasses in the last 4 years and they can't get my left eye to see 20/20. It's close, but I can definitely see better with my right eye. I'm pretty sure it's because my left eye has an astigmatism..but it could be something else, who knows? I got my last pair a couple of months ago at Terminal 21. I was very thorough with the exam, telling them my left eye wasn't seeing as good as my left..they got a little flustered and suggested I might have to see an eye doctor. I ended up paying 6,000 them for a pair(it took them 30 minutes lol)just to see if they could make it any better than my other 2 pair at home. Turns out it's about the same, my left eye just a little blurry/out of focus.
I'm going to see an eye doctor here in Pattaya, she's pretty good maybe it's just an astigmatism and it can be corrected but I'm 58 and I've been wearing glasses since I was 9 and contacts since I was 17 my eyes have been through hell I might have to manage expectations. I do have confidence I'll at least find out what's wrong.
Maybe some of this info will help you out. My best suggestion is to go to a different place and tell them your situation, maybe it's your eye(s) and not your prescription.