After reading through a few forums, I observed a trend for people who got stuck with their I-485 approvals. I think it has to do with the 3rd world level databasing technology used in USCIS (disclaimer: this is pure personal opinion without any solid knowledge on the processing procedure):
1. People apply I-485 before PD becomes current: I'd imagine that these applications go to the "Not ready to be processed" black hole. When PD becomes current, someone might dig out the applications from this black hole. Purely depends on luck!
2. Similar to 1, concurrent filing, again, your I-485 goes to the same black hole, when your I-140 gets approved, it depends on your karma whether your I-485 gets fished out from this black hole.
So, I think the best way to avoid your I-485 sink in the black hole is to submit it when your PD is current and after your I-140 has been approved. So when your I-485 arrives at the mailroom, it will go to the "Ready to be adjudicated" pile, hopefully if this pile actually exists.