iPod touch music exclamation mark

I have a 5th gen iPod Touch with the latest iOS. This morning when I went to work, I wanted to listen to some music and the iPod said, there is no music at all on my iPod. This happened previously one or two times, I got a bit angry, but I restored my iPod and put some music back. (I ALWAYS MANAGE MY MUSIC MANUALLY)

Today, when I got to my workplace, I connected the iPod to my laptop and all the songs where visible. As I clicked on them, one by one, the well known exclamation marks appeared. I got really angry and restored my iPod, I gave up on fixing the problem and I rather put new music on it, just so it works.

So I restored the iPod, put music on it. When I checked that all the music is uploaded, there where some songs in grey, with the dotted circle where the exclamation mark was before. I searched for these songs manually, dragged them to the iPod and the dotted circle filled up with grey colour clockwise and they where there as the other tracks.

Then when I was coming home and wanted to listen to my iPod, only those songs where on it, that I manually pulled one by one, which had the dotted circle next to them previously.

Now I am home, connected the iPod and all the songs have the exclamation mark except the few which has been carefully added later because they had the dotted circle around them.