Ruth,
Your 6yo is pretty young and new at this. Yes, it CAN be hard. It takes many children a while to get this down. Now, about details. You may very well find that one sentence at a time is difficult to track and do. I don’t think Aesop’s Fables are too hard, but you might need to think about approach. Sentence, narrate, sentence, narrate, is a LOT of interruptions to the flow of the story. Try reading him a whole story and have him draw or act out or something as you go. I’ve looked at a story, seen what we need, we drew a quick fox or whatever, stuck him on a popsicle stick if we were feeling fancy, and AS I read, he would act it out or show me or draw it. THEN at the end I’d have him retell it. This is how I helped my third son. Another thing to try is to do a short strip for each sentence–a few basic words or a picture if you can, for a sentence OR a single act in the story. Have the child find that strip or picture. THEN keep going. AT the end have him retell the entire story. Asking lots of questions can frustrate a young child. Reading the same sentence more than once also tells him that he doesn’t really need to pay attention the first time.
All children aren’t automatically ready at age 6, but before you assume that you can try some different things like the above.
ALSO, try telling simple stories yourself to see if he’s paying attention. A three-step story can be best. Make them fun, based on his interests if you like. For my youngest it was always dinosaurs. T Rex did this, then did this, then this happened. I’d then say the first part again and ask him what happened next. Or I’d describe what I was doing–I’m going to fill the sink, then wash the dishes, then we’ll dry them together. OK, after I fill the sink, what’s next???