If there are multiple spoiler tags in one post - including quotes - then only the first one by the poster works. All quoted spoiler tags will work, but the current poster can only add one new one, and if it's after a quoted spoiler, it won't work.
Occasionally getting issues where multiple spoiler tags are causing display issues to the point that one of my posts appeared to disappear until I managed to hack the edit page and make it one spoiler tag instead.