Make sure you are not trading those resources to someone. And I'm not sure about thise but I think you only get bonus for EXTRA resources, meaning if you have 2 iron, then you only get 1 hammer bonus because the other iron is "used" by your civ.
Edit: another thing, I think you get 1 hammer per 2 extra resources.
Edit 2: Ok nm, just went to a save game and checked, have 14 resources total, and corp gives +14 hammers. 4 iron, 4 copper, 2 silver, 1 gold, 3 coal. SO yeah check that you are not trading the resources away.