It's either because....

• I checked your page and it's not active
• You don't post things that interest me
• I forgot to
• I don't know you
• You followed me in my Tumblr art blog (can't follow back from there)

Anyways, don't take it to the personal level if I don't follow back/unfollow- these are just social medias and you won't die from not having me following you -w-