Certain local files won't download to my Samsung S6 edge

[GreenAndPurple]

I am trying to put some songs that I have that aren't available on spotify onto my phone, most of them download albeit with a lot of convincing for some of them (having to make a new playlist just to download one song, changing the names of the files) but a few songs really do not want to download. I even cleared all my downloaded songs and now some of them that used to be downloaded won't download again. It will occasionally flash a message saying that I've discovered a premium feature when I already have premium. I am so confused I just want to get some songs on my **bleep** phone.