The GM's could have kicked out a player from their campaigns. That resets the links. The GM's can also rescind the invite links that are not used at any time.
Two things you can do. One is pm the GM's and ask for a new link or post in their looking for player listing in that campaign (you can do this if you know the campaign name or have the url). If they are fresh invite links that is giving problems, a workaround is to post in the looking for player listing so the GM can do a direct invite.