Music not appearing on iPhone

I am having a reoccuring issue with music bought both on iTunes on my iPhone and on my computer and that is that many tracks and even complete album does not appear on one or both devices.

Latest issue is an album where almost all but 6 songs appear and when I go into iTunes to try and download these tracks again, I am met with 'Purchased', so cannot do anything.

Some times the tracks will appear by themselves or it seems to happen when I shut down my device, but this time all things I have done seems to have no affect.

Please help, I have seen some answers including going into settings and signing out and back in, I have also done this, but nothing helps.