Hi, DavidRealEstate,
It may be that the songs that are not visible are DRM (Digital Rights Management) protected and therefore cannot be used in iMovie. Music purchased from iTunes before year 2009 carried DRM. Also, all music purchased from Apple Music is DRM protected, and downloading the songs to iTunes will not remove that projection. Those songs cannot be used.
iTunes has a Match program that you can subscribe to for about $24.97 per year that will let you download from the Cloud DRM free versions of you DRM protected songs. That would work for songs originally purchased from iTunes but I don't think that it will work for songs that involved Apple Music in the chain of purchase.
The easiest way out of this may be to repurchase from iTunes whatever protected song that you want to use in your project. iTunes discontinued DRM protection in year 2009.
-- Rich