Here are some straightforward reasons why you might be getting a low follow back rate:
1. You are an egg - using the default picture of an egg rather than uploading a picture makes you look like spam or a real beginner who will probably make an omelette of their tweets. You don't have to use a portrait photo if you don't want to, just don't be a huevo, because the only one anyone ever heard of was Humpty Dumpty and he was so smashed he fell off a wall.
2. You have a) no biog b) an annoying biog - don't make people guess what you're about. Give them a hint, set the tone, so they know why they should follow you. That said, having no biog is probably better than a bad biog, as the saying goes "Better to have no biog and be thought a fool than to have a cringeworthy one and remove all doubt." Best not to say I'm so crazy!!!! or I'm so funny!! LOL, writers are always saying it but in this case it really is a matter of 'show don't tell'.
3. Your ratio of following to followers - if someone is following 1000 people but only 12 are following them back, it will make me suspicious. Since roughly half of people follow back, why have only 12 done so? How bad can the person's tweets possibly be? It's normal for people to be following more than they are being followed by, but not by such a staggering difference.
4. Your timeline is full of the same message - if I'm not sure whether to follow someone back I might have a peek at their timeline. If I see that all their tweets are identical and run along the lines of 'Thank you for following! Check out my latest 5 star review of my new novel: Tedium Strikes Back...' that's enough evidence for me that that person is only on Twitter to bombard me like a robot.
5. It's not you, it's them - if you're not an egg, do have a reasonable biog, are following and being followed back by more than just your nan and aren't doing the hard sale, then don't worry about it if some people don't follow you back. Some tweeps are very choosy, or famous, or like to keep the group they follow small so they can manage their twitter feed more easily. They might not share your interests, hobbies, have an aversion to cat/dog/chinchilla people or think because you've written you're a Christian you're going to be constantly tweeting extracts from the Bible. It might be simpler than that even. Some people might not have got around to checking out any new followers, in which case retweeting a tweet of theirs, is a good way of alerting them to you. You can use friendorfollow.com to investigate further. It will show you who you are following, who follows you and who you are following who doesn't follow you - now have a couple of drinks and try saying that sentence very quickly!
For blog tips my ebook Blogging for Beginners is available to download from amazon.