What does 'breast sit up' mean?
Its best to post a photo with your question if you wish to get the response from the gallery of surgeons here.
Breast Augmentation Dissatisfaction
Unfortunately, your description is ambiguous and you do not provide photos. Please repost, ask your surgeon, or get a few second opinion consultations with ABPS certified/ASAPS member surgeons.
Best of luck!
Without photographs to see how your breasts normally sit, I am not able to answer you specifically. However, some women do have normally low breasts that are not sagging, and some women have low sagging breasts. Implants do not rise up higher over time unless scar tissue or capsulation occurs. I would suggest asking your plastic surgeon about any concerns you have. Perhaps a lift would be helpful.