I agree that its just a lack of confidence out there sometimes, but I also think OSUBJ hit on another big thing- at times, it seems like he is reluctant to shoot too much, even if he is hitting his shots. It seems, to me at least, like he doesn't want to be seen as a ballhog (by his teammates and coaches) who will shoot whenever he touches it. It almost looks like he needs someone, and I think it needs to be one of his veteran teammates like Lighty or Turner to come over to him and say, "don't worry about how many shots your taking, we know your a great shooter, if your open take it and if you miss it happens we wont think any less of you for shooting when your open." Now they very well may have said this to him already, and if not I hope they do say this to him because I feel like thats what he needs right now is confidence, and that is a surefire way to build his confidence in his shot back up.
I have said it a million times- I am continuously amazed with Diebler's basketball IQ. However, I think at times it gets him into trouble, because I feel at times like he is thinking way too much, trying to make the extra pass too often, instead of just playing loose out there. It seems like, on some of the shots he is missing, his eyes are darting all over the court until he has already elevated to shoot, rather than him realizing he has an open shot and concentrating on taking it and making it.