I was told many years ago when we had a roach infestation in our apartment that if all the apartments in the building are not fumigated at the same time, you will always have a roach problem. When your apartment is bombed, they will hide in another apartment or between apartments until the spray fades away. Eventually, they'll be back.

Oh, and speaking from experience...make sure if/when you move, that you spray everything or you'll take them with you and the problem will continue.