About the only way that can happen is if you have updated Motion to a newer version than the version of Final Cut you have installed can handle. Final Cut Pro is capable of running all Motion templates from it's version's release date backwards, but not forwards. So, for example, if you have Final Cut 10.3 and Motion 5.4, the templates created in the newer Motion will appear as "missing" in FCPX. (To make things even more problematic is the subversions of FCPX and Motion haven't always "matched" so it's a little harder to tell exactly which versions of Motion are compatible with seemingly comparable versions of FCPX). There's a compatibility guide here:
https://fcpxtemplates.com/fcpx-motion-compatibility/
at the bottom of that post (there's also instructions how to *backdate* a template, if possible, to make it compatible with an earlier version of FCPX).