Some songs won't transfer from iMac to iPad library

Both my husband and I have run into this for years. I have many songs on my Mac that never sync to my iPad (and no error message, they just aren't there). One Apple advisor said "you don't expect ALL your 8,000 songs to tranfer do you? It would take up too much memory on your iPad". The next Apple advisor worked with me for about 30 minutes and we gave up. Now today my hubby has a song downloaded from Apple Music to his Mac and needs it on his iPad, so our band can play it. Has anyone ever come up with a reason for this and a way to resolve it? It is SO frustrating.