Oftentimes people will unfollow you if you do not follow them back. There is a myriad of software and services which allow people to automatically un-follow people who do not follow them.
If you are following these people and they are still unfollowing you, chances are there is something that they do not like about your content sharing strategy. Make sure that you go through your follower list and re-tweet worthy content. When you re-tweet their content first, they will be indebted to you and will re-tweet you in return.
Finally, when you do tweet, try to limit your self-promotion tweets to 1 in every 15 tweets. The rest of your tweets should be re-tweets and shares from other areas in your industry.