I wouldn't pitch that egg just yet, either. Sometimes they surprise you. I've had a few that I was sure were late quitters that wound up hatching just fine!
I usually wind up with eggs with various due dates all in the 'bator at the same time, so I can't do a lockdown. Mine usually manage to hatch anyway, there have only been a few that I wound up assisting. I have a little Cochin egg that wasn't due until tomorrow that surprised me by being pipped and cheeping this morning.
Even the 19 days of Serama incubation can seem like an eternity, can't it? I'm waiting on a few Serama eggs of my own, one due every other day through next Sunday. So far, I am 2 for 2, with another due to hatch on Saturday. Sending happy hatching thoughts!