Some songs are grey and won't play on mobile, but will just fine on my computer?

[Dud3ManGuy]

I uploaded a lot of local files from my PC and some of them will play with no issue on my Android, but some of them are grey and wont play when I select them. All songs will play just fine on my computer. 

EDIT: Fixed a typo

[Joe]

Is it possible that the songs which will play are available on spotify and the ones which won't play aren't? If so, it sounds like your local files haven't downloaded to your android.