The most common reason is protections of the target that were not accounted for causing reflection back to you. Or just a stronger will than you. It’s usually recommended to use divination to find out what happened exactly and account for it if you intend to repeat the working.
Something that people miss when we’re trying to find out own abilities, is that other people all have abilities too, and you don’t need training to use them, healing and protection come naturally. Never underestimate a target for baneful work. 
Edit: Actually this post over here synchronistically looks like something you want to have a look at: