In reply to mike1979:
Totally agree with you about bouldering and doing too much relatively easy stuff. I feel the same about the next colour code up at my local wall. It seems desperate and too out of reach. However recently I went bouldering with a friend who's much better than me and I ended up on stuff I wouldn't normally go on. I didn't get up anything but I could definitely see with some work I would be able to do
some of the harder problems over a few sessions. I've used this approach in the past and definitely got stronger that way.
So I think you have to force yourself to get on the harder stuff, even if it means not getting up anything for a while. Break the problems down. Can you do all the moves? If not then how far off those moves are you? Can you touch the next hold? Or how far off are you? You have to try those moves repeatedly. And while trying you have to really psyche yourself up to try really hard. There is a mental aspect to trying really hard, just like any high intensity exercise. It also helps if you can see other climbers do those moves, or even better chat to them, so you have a good idea that you using the right technique and you're not missing some subtlety. Once you know what to do you can really concentrate on giving a move maximum effort. Even if you don't do it, think of trying it repeatedly as kind of specific exercise. When you come back a day or so later you'll be slightly stronger at that move. After a few sessions you'll hopefully be able to do it so then you can work on the whole problem. If you do this with several different problems you'll be getting stronger/better at all of them. If you finish the session and you feel worked, maybe some muscle soreness the next day, you know you've had a productive session, even if you didn't get up anything.
I'm no expert on campusing but I'd have thought general exercise principles apply. You need to change what you're doing. Increase the volume, change the exercise, train more days per week, or increase intensity. There are loads of different exercises you can do on a campus board so don't just stick with 1, 3, 5.