[It -Stephen King - Book] Why didn't Pennywise transform to something different when trapping Mike in the smokestack?

It had him trapped dead to rights and was too big to fit in the smokestack to get him properly. IT can change form at will. Why didn't it? I know Mike was scared of the bird but he's also scared of the 'ghosts' of the Ironworks kids who died and probably many other things.