I have a theory on this bug, which happened to me a few times as well.
What I think happens: if you are logged into iTunesConnect, then the web page will query the database for available builds. This querying could possibly interfere with the creation of new builds.
What I did was:Log off from iTunesConnect on all your web browsers.Increase the build nr in Xcode, make an Archive, and upload it.
Then: resist the temptations to log into iTunesConnect to check on the status. Wait at least an hr before logging on.
When I did finally log on to iTunesConnect, I found my binaries sitting there, and waiting for me.