can't see the wood for the trees

can't see the wood for the trees  (British, American & Australian) also can't see the forest for the trees (American & Australian)

if someone can't see the wood for the trees, they are unable to understand what is important in a situation because they are giving too much attention to details After you've spent years researching a single topic you get to a point where you can't see the wood for the trees.

See also: see, tree, wood