Have to keep re-downloading music purchased on my phone?

I have purchased several albums from iTunes on my iPhone. Every couple of days they go to a light grey colour and I have to re-download them, I dont have to pay for them again but I dont have unlimited internet so it eats up my data. I have tried transferring the music to my laptop and then adding the songs to the playlist for my phone and then syncing the phone to the computer but this hasnt worked. Anyone have any idea of what I can do to keep this music on my phone without having to download them again every time I want to listen to them?