Q: I am trying to add music from my iTunes folder but iMovie seem to only pick up a few tunes and not my tune library.  It is iMovie 10.1.2. Any ideas please. Harry

Is your music in the cloud? Then download the songs in iTunes, so they are in your library, before you try to add them to a sound track.

Adding to what Léone said, songs in your iTunes library that are DRM protected will not show up in your iMovie iTunes

folder.  Look at the bottom of your iMovie iTunes folder display and see if it says that "___ files are protected and unavailable."

If that's your issue, these songs would not be usable in iMovie even if dragged in from a folder in the finder.  They would be ones that you purchased before 2009, when iTunes had DRM protection on its songs.  Since then iTunes has discontinued that practice. 

Since then iTunes has discontinued that practice.

That is what the iTunes Store claims. But DRM songs keep appearing on collection albums again and again.

I have quite a few of them with a purchase date newer than 2009.

For example: Purchased in 2012:

An btw, Apple Music songs will probably also not work, because they are protected as well.

  The ol' difference between word and deed.     Bummer.