A good marking dog is born that way. They just know where they are going and can count. You know it when you see it. You can enhance marking to a degree but you can't really may a poor marking dog into a good one just by more marks.By good marking dog I do not mean what they say in the classifieds " great marking dog.Runs blinds like they do marks". Watch dogs like Grady, Ivy, Ford and others. That's marking ability.
To the OP I would take the suggestion and have the eyes checked. If ok then your pup should still be a good hunting dog. I had a very well bred BLF that wasn't up to FT marking but she is a very good hunting dog and got a great home. She could count but not at any FT distances.