I think I understand what you are saying with your remembering these details but the other person seemingly not.

The trick is to remember that you get to pick the details you remember in your head whereas the person you are remembering about has a whole slew of other details they are remembering in that time period between conversations with you.

I read somewhere that the average number of people we know well enough in a life time that they would come to our funeral is 250. So, while you might remember 250 items about those 250 people, each of them is remembering a different 250 items about 250 different people, you'd only be 1 of those people?

So, you are doing well being able to remind them of more of the conversation that interests you! Some people will remember and pick up on the conversation and others will not, people have different degrees of memory. I thought for sure your purple hair grocery clerk would remember the blue hair and was disappointed you gave up on her and just said "thank you" and left :-) I wanted to know more from her.

Sometimes I find it is fun to pretend you have not had the conversation before and ask the same questions you asked last time? You could have asked Purple Hair, "Have you only tried purple hair or do you change colors?" and see if she mentions the blue and then ask about shade of blue: dark, midnight; or bright, electric?