Your spin is not enough. Spin harder.
just remember these steps.
1. curve just a pinch cuz if you carve too much you'll fall face first if you carve to little you wont get the full 180 (when you get better you'll stop carving as much)
2. turn your head over your shoulder this will help your turn out a lot and it will look a lot cleaner
3. tuck the bike so you get higher and this ill come in handy when you try your 180 up something big
4.try your hardest to land straight or else you'll land to much to one side and it will just feel sloppy and look sloppy plus just impossible to land
5. pedal backwards and pivet when you get better you'll learn half cabs, full cabs, fakie half cab barspin and so on