We've been getting large black house flies in our apartment. Help!?

5-10 large black (probably) houseflies have been getting into our 3rd floor apartment everyday for the last couple days. We think they might be getting in through a space between the sliding gl doors to our balcony, but we can't pinpoint the exact spot. We keep our apartment clean and don't know what could be attracting them inside. How can we get them to stop getting in?