I'm assuming the following:

1. On my 15-Sport, the castle nut is staked to the end plate. If yours was staked in a similar fashion, I assume you had to peen back the staking or grind back the staking to get the castle nut to move.

2. When you reinstalled the buffer tube, you correctly positioned the lip of the tube to retain the buffer retaining pin.

3. Peening back or grinding the staking can ruin the castle nut(?). If it did, you replaced the castle nut.

4. You used a castle nut wrench to tighten things down.

5. When it was all tightened down and in place, you used a screwdriver & hammer to stake the end plate & castle nut in place to keep them from vibrating loose.

The alternate is to use a dab of blue thread lock on the buffer tube. Just remember that to break it free later will require a little heat.

Now if all of that was done, then inspect the lip of the buffer tube that is supposed to retain the pin. Is it chipped or damaged?