Tactics Training Trials and Tribulations
Fun, frustrating, fruitful--that's how I'd describe the Tactics Trainer program on this website. It's a great way to accumulate a database of common chess problems and how to solve them.
As you work your way up in the ratings the problems get more abstract and usually more difficult, though once in a while you'll get one that's so easy, you miss it because of analysis paralysis. You think so hard about it, but the solution is a very visible pawn nabbing the opposing queen.
I find that there are days when I can solve almost any of the puzzles offered up. More often than not, though, I struggle to get some of them. My ratings show a gradual increase or decline. Then there are days when I can't think my way out of a paper bag and NONE of them seem easy. Then it becomes this black hole where I see puzzles I've already done several times, and rather than trying to solve the problem, I try to remember the solution.
Another problem is that if I lose a hundred points or so, the problems become more straightforward and less abstract. Again, analysis paralysis prevails, and the forest disappears in the trees.
Your score really shouldn't be the focus. If all you do when you fail to get a problem is say "d######," and click "next problem," you aren't getting the most out of it. If you miss a problem, go back over it a couple of times until you understand how it works and why the solution makes sense.