At tray 20 from 29, why did k-9 tooth stop tracking. After 28 trays I've got 41 refinement trays.

This is a relatively common question.  Canines have very long roots.  These roots make them extremely difficult to move with Invisalign and most adults don't want to wear unsightly metal braces.  This is why I suggest having your cosmetic dental work done at a cosmetic dental office.  Reason being, a cosmetic dentist would simply recommend placing a veneer on that tooth in one appointment to make it permanently appear rotated, rather than trying for months or years to rotate it.  Rotating a canine is like trying to rotate a shrub or oak tree; it can be done, but it is much more efficient to just trim it and change the appearance of the sides that are in view.I wish you best of luck with your disobedient k-9.  Follow me if you have any more questions.Take care.

Teeth don't track well for two reasons .  Collision ( meaning it touches another tooth) and compliance (you are not wearing your trays 20-22 hours per day,etc).  Obviously during your treatment your canine stopped tracking.  IN order to get it to move a refinement was probably done which is longer than the original case.  This is normal and happens sometimes.  Good luckHope this helps