Festus is trying the best he can but he is no match for the better big men in the league. Lee is a finesse PF who lacks hops and provides very little defensively. Landry is also primarily an offensive player. AB is a decent rebounder but also is average defensively.
Bogut is the missing piece to the puzzle. He is a top notch defensive C. So its no surprise that the W's struggle against strong front lines without him.