This is something to do with how the site is programmed, and we can't add pieces retroactively to vote. I don't know enough about web development to explain in any more depth, but us moderators don't have the permissions to change this kind of stuff... it would help a LOT.
I would also like to suggest to sedgemonkey to colour code different kinds of submissions (or at least label them clearly) on the console page, as we currently have to manually check (as in visit the piece's page) if a piece has been ticked as a Weekly once we've ascertained it is indeed a Weekly piece.
I've seen this happen a few times, and have wondered:
is it really impossible to add an entry to the poll if it was missed during the week, or is this a decision made on the basis that the brief period the unincluded piece was not available means it will not get a fair share of votes from people who voted before its inclusion?
Please check out chimp_chomper's piece, You woodn't miss the first day of school... wood you?