An expression used of someone who is too involved in the details of a problem to look at the situation as a whole: “The congressman became so involved in the wording of his bill that he couldn't see the forest for the trees; he did not realize that the bill could never pass.”
can't see the forest for the trees
Also, can't see the wood for the trees. Focus only on small details and fail to understand larger plans or principles, as in Alex argues about petty cash and overlooks the budget
he can't see the forest for the trees. This expression was already a proverb in John Heywood's 1546 collection.