Breast implants under muscle keep slipping out of pocket

I had a breast lift with 300cc saline implants placed under the muscle in 12/2023. They were fine initially but by 6 months post-op, the right implant had slipped from its pocket down and toward the armpit. You could see the implant bulging through the skin and the right nipple was significantly higher than the left. My surgeon performed a pocket revision in 9/2024. It’s now 11/2024 and the left implant has slipped from its pocket in the same way. Why is this happening and what’s the solution?