Music not syncing to iPad and iPhone

What problem do I have?

1. Some tracks are not downloading to my iPad and iPhone

2. Some artwork is missing

What have I tried to do to fix the issue? (in no particular order)

1. Deleted all the music > sync > recheck all the music > resync

2. Checked and unchecked "Show All Music' in Settings

3. Logged out off iTunes and iCloud and logged back in.

I can see from some forum questions that this appears to be a common problem but can't find a solution.

Any ideas?

Thank you.