Its very difficult to say why we dont catch sometimes. I'm sure most of the people on here have had some really frustrating times with not catching. Everything may be right and it still doesnt go your way, thats carp fishing for you! What I would say is look at all aspects of your fishing and just experiment with different things such as length of hooklink, bait size, but dont forget the most important things such as location. Just keep at it and i'm sure the results will come. Hope that helps in some way.
Good luck!
Jonboy