Purchased Songs un-downloadable

How can i get back the grey labeled songs in picture below?

A few Songs in an Album i bought on iTunes aren't downloadable anymore to my iPhone. This songs are missing and it looks like this in iTunes for the iPhone 6 on the latest Software :

So i checked:

• if the purchases are hidden > they are not
• if i can find the purchases on my MacBookAir iTunes > they are completely there
• if i could sync the songs from my MacBookAir iTunes > its not possible
• if its working if i delet everythin on my Music from my iphone and then sync everything again from my MacBookAir iTunes > ist not possible
• if its possible to try to drag n drop it manually to my iphone from MacBookAir iTunes > its not

If anybody can help i would be really appreciate it. Many hours of trying haven't worked out so far.

Thanks in advance