Manually added songs to iTunes on Win10 do not show up on Ipad Music (with iTunes Match)

I bought a series of "Play Along" books from Hal Leonard. These books were accompanied by a CD in the past but these days the play along and backing tracks are to be downloaded first and then they can be added to iTunes (-I am running Version 12.11.0.26-). On a Win10 PC using the normal "Add File to

Library" these songs can be added to the iTunes library. This works perfectly. All songs from each of the 4 books that I bought were added and each book is correctly visualized as an album (with the correct ArtWork) on the Win10 PC. All files are stored at the correct location in the iTunes Media folder and each song can be played on the PC. None of the songs has an error or warning icon.

However, none of the songs or books show up on the Ipad in the Music app: not in the "Recently Added" library and also not under "Albums" or "Songs". I bought loads of music on iTunes Store via Win10 PC and immediately after download on PC these purchases were visible in "Recently Added" library of the Ipad. This is not the case now with the manually added songs; so it seems that

the sync with iTunes Match is "broken"/not working for the songs that were added manually. Strange enough when I buy a song from the iTunes store (-which I did a few seconds ago-), that song is instantly visible on the Ipad!

Any idea what is going wrong here and how to fix is? I also have an iTunes Match subscription.

Thanks for your support!