Bottoming out can happen. There are several reasons that it can happen. One is overdissection of the pocket, another is a heavy implant, still another is poor quality tissues. These are just a few causes.
Tissue relaxation may cause breast implants to bottom out
Implants could bottom out for a variety of reasons. Some are related to the physical characteristics of the breast tissue that relaxes with time under the pressure of a heavy implant.
Heavier implants and over-dissection would lead to the same problem.
Dear Gingercat, Please check my answer to your other question. You should seek second opinions if you have lost confidence in your surgeon. A good doctor will want to get you to your best outcome and hopefully yours feels that way.