A solution for those that have an unobtainable Pivot in a building/ rock, etc. (Credit goes to Darsh D on Youtube, playing on a PS3, using a solution developed by Adam Gulch) (THIS WORKS FOR 360!).
I unluckily had a Pivot (my last one) stuck in a building (no way in) in southern New York. For days I tried everything until I saw a video on Youtube of a guy playing AC3 on PS3 solving the problem of the unobtainable Pivot and adapted his solution to this one:
Here are the steps to re-sync that problem Pivot to a different (obtainable) one that worked for me moments ago.
1. Place all remaining pivots (11) in a circle around unobtainable Pivot, making sure every line intersects problem Pivot (on your map there should be no set pivots without a line through them).
2. MOST IMPORTANT: Go into Pivot info screen and HIGHLIGHT problem Pivot with your cursor and LEAVE it there! (for me is was pivot Sigma). STAY IN THIS PIVOT INFO MENU! Do NOT do anything else except the rest of this solution OR this will NOT work!
3. UNPLUG the power to your modem/ router or Ethernet cable (I unplugged modem). (to disconnect with UBISOFT servers).
4. In a few seconds a warning box (disconnected from UBISOFT servers) will pop up. Reconnect internet signal. (I powered back up modem). (forces a resynchronization with UBISOFT servers).
5. You should have the option to hit the Y button to reconnect. Push Y and sign back into your profile.
6. Immediately when you get control of your cursor back, highlight problem Pivot again.
7. This resynchronization should now have rescanned a NEW Pivot for you to find in a new location! (Frontier for me). If this doesn't work the first time keep trying. (This worked for me on the FIRST try).
8. If you followed these steps very carefully you can now go collect your newly scanned last Pivot, Sync To The Cloud, and enjoy your achievement!
I hope this truly helps someone as I have probably never been more mad/happy/relieved all at the same time, which, in turn, prompted me to write this solution. Good luck.