In reply to SuperstarDJ:
I doubt it's a question of strength but even if it is, you will naturally get stronger by climbing. You can try overhangs with big holds.
My guess is that you are not as relaxed as you should be and you are trying to hold your body weight with your arms. Trying to relax the arms and try and twist by keeping them straight, hopefully you'll feel your shoulders working too and also your core.
Is your footwork right? It would be the number one thing to check along with your body position before assuming it has to do with lack of strength. Can be impossible to hold some lock offs if your body is doing the wrong thing.
You could also try climbing frontal on an inclined wall for strength - good for stabilising the shoulder in open positions.
I guess for lock off strength you could try negative reps on one-arm pull ups. If you can't do it you can try having the other hand holding onto a rope lower down, if that makes sense?
You could just campusing up juggy routes too...