Damage to the buffer retaining detent pin is either done by the carrier or the buffer.
When you close the upper down on the lower, can you feel the back of the carrier pushing the buffer slighty back? If the buffer is making contact with the pin on the cycle stroke, then the midle post gets bent.
If the carrier is doing the damage, then the tip of the pin is either too tall and making contact with the bottom of the carrier, or the bottom slot sides on the carrier has a bur that is chewing the sides of the retaining pin.
If you let use know where the/wht kind of damage is being created (chewed up or bent), then we can help you correct the problem.