One of my Implants Has "Slipped" out from Under the Muscle

I would discuss this with the surgeon. IF you have already done this, your next option is to discuss the revision and possible charges. Alternatively you could get a second opinion to confirm your suspicions.

Although all plastic surgeons would like all of their cases to  have a perfect outcome, it doesn't happen. The reasons may be many including incomplete release of the lower portion of the muscle, where it attaches to the ribs and lower portion of the breastbone or it may be that the overlying breast is not satisfactorily supporting the weight of the implant. The other possibility is that the dissection on that side was carried too far below the inframmary crease but that would be something, that you would see immediately.

In patients that I see with your problem, the most common reason is that supporting structures of the inferior half of the breast don't support the weight and give way. In this situation I will re-operate on the patient and reinforce the inferior pole ,either using tissue within the breast or  adding a graft made of synthetic dermal matrix. Always consult with your original surgeon and get his opinion. If you're not comfortable with the answer then you should seek a secondopinion

"Bottoming Out" the inferior slippage of breast implants are often associated with progressive loss of superior muscle coverage of the implant. This can be caused by several factors acting alone or together. Usually any weakening or over-dissection of the lower aspect of the implant pocket combined with large implants and failure to wear support could result in inferior implant movement with time.

Correction would need to be an operation in which the lower implant pocket is strengthened either with sutures, or much more reliably with an acellular graft sheet (strattice or Alloderm). Discuss this with your surgeon.