The issue could be that the Story Projects are internally referencing a version of the story format that is no longer installed. To fix this issue try opening the story project and changing the story format to one of the others (that you don't want) then change it back to the story format you want to use.
a. You may need to close the "Story Format" dialog or even the Story Project itself between the selecting of the Other story format and the selecting of the Wanted story format.
b. The Twine 2 application contains code that is mean to automatically update all story projects to the latest installed version of each series/brand of story format, but it may not of worked in your case for some unknown reason.