Sometimes a show, studio or channel will pull their streams down from the internet when their marketing team is pushing sales through any streaming service like VuDu, Redbox, their website, Hulu etc.
When the online sale ends the links will go back online and your OnDemand section will allow you to watch them again.
There isn’t anything we can do to avoid that. So something you have been waiting to watch may not work for a few days to even a few months.
It’s a small price to pay for the ability to watch so many movies and shows from the comforts of your home.