Did you tell him there was a gap when you were with him? See, the denture may have to be re-done and no one wants to do that. But that's what you're paying for; dentures that fit you nicely, that are custom made for you.
It's all in their LAB. The lab works for the dentist and the dentist must see to it that the lab makes the dentures per his mold of your mouth. Sometimes, dentists think, "he's imagining things; soon he'll get used to them. " NOT.
Just tell him, in person, that the uppers fall down and they just don't fit. Be confident and firm. And good luck .