Without an examination or at least a photograph, it's impossible to know why better symmetry cannot be achieved. Sometimes, no matter what is done, a perfect match is not possible. Having said that, in my experience, fat grafting after achieving maximal symmetry with lifts and implants, has been the most successful modality in trying to match the two breasts. Good luck.
It is difficult to say why your result shave not improved. Perfection and perfect symmetry are never achievable.
Without knowing your exact history or having a picture to reference it is hard to answer. If you had a difficult pre-operative breast shape (tuberous breasts) then multiple stages or revisions would not be surprising. If you had average breast shape, nipple position, and symmetry before surgery then multiple revisions would be out of the range of normal.
I'm sorry to hear about your concerns after breast surgery. Unfortunately, as you can imagine, every patient's situation is different and therefore is not possible to generalize about how many operations it may take two achieve an outcome you will be pleased with. Best to seek consultation with well experienced board-certified plastic surgeons who can demonstrate significant experience helping patients with the type of revisionary surgery you are considering. Then, communicate your goals carefully as well. Your chosen plastic surgeon will be a much better position to advise you more specifically. I hope this, and the attached link, dedicate to revisionary breast surgery concerns. Best wishes.