Good thing it worked now.Mine was on my iPad, my phone has been fine? It kept telling me the server was unavailable or my password was wrong, after many many times of trying I ended up going onto the yahoo page signing in there, changed my password, and tried again to add the account on my iPad and it worked.
Do you have the yahoo native app or are you using Apple iMail? I use iMail for Yahoo mail and have no problems.