Keep re-downloading songs from apple music

Hi~~ I am a new user for the apple music thing, when I enter my music library in my ipod touch, I search for songs that I like and add to my music library, since ipod touch doesn't have data, I then download the songs that I had added in my library, it works!! However, from time to time the music is gone and I need to re-download every song (The cloud icon thingie appear again. If it is saved on device, the icon will be a "tick on an ipod"). I have download all the songs to my mac via itunes, can I just sync the whole library to my ipod?? I tried and it doesn't even give me an option to sync those songs. I am so annoyed that I have to download all the songs (50+albums) like every 1-2 weeks... Can someone help me with this?? Thank you very muchhhh!!!