I've found the most common mistake is too big idle gap and compensating with a rich lsn, with a big idle gap you will be chasing a tune all day and cause a two stage idle, if your idle gap looks about right and you think your tune is close more than likely could be a possible air leak if the two stage is still there.
I still learning myself but couldn't count how may times the rich bottom, lean top, too big idle gap has caught me out, best to set idle gap reset needles and start again with the tune.