Put some silicone caulking behind the flange, push it up flush against the ceiling, and use painter's tape to keep it in place until the silicone sets up.

It doesn't go flush but maybe around the part that touches the ceiling. Good idea. Thank you.

I suggest you put the flange on the pipe and see if you need to bend it slghtly to make the angle of the flange match the angle of the ceiling before you move it into place. Usually the hole in the flange is slightly oversize, the metal is thin, and there are little tabs in the hole that are bendable so you are unlikely to distort the flange when doing this. If you still have a problem, post a picture.

Not sure why the flange doesnot sit against the ceiling maybe post a picture

Oh kudzu9 I just saw your second response. I'll try that. Thank you.