How are you using it? Whenever you get a puzzle wrong you should look at the solution. And don't "guess" what the solution is likely to be. Try to calculate everything out.
IMO Puzzle books are better than online tactics trainers due to being able to do the same puzzles again, but not many people seem to agree with me so there must be something I'm missing...
It certainly wouldn't hurt to know the names of the themes, but you just have to burn them into your brain by solving lots and lots of puzzles, and for that, online tactics servers are best.
I've been doing the tactics trainer since I got here, and am barely improving. Any suggestions?