Why Ran did not recognise Shinichi right away?
She may well have noticed the resemblance, but seeing an elementary school student who looks a lot like how a friend of yours used to look in the past, does not immediately lead one to the conclusion that that student is in fact your actual friend! It is never explicitly stated, but in all likelihood she simply noted (subconsciously?) the similarity (if she were paying attention in the first place) and moved on.