When you parlay, some books count a push as a loss. In that case, don't place any parlays with that book. There are lots of books that do it the "right" way and count a push as a push/no-bet.
I personally don't know of any book that will grade a push on a parlay as a loss. If you have a 3 teamer, 1 push, it becomes a 2 teamer with the other 2 and the correspondant payout for a 2 teamer.
most sports books use these guidelines......be careful when making an "open-ended parlay"......ie wiscy/unlv game that was cancelled two years ago...left many players in the dark..(pun intended....lol)