rear derrailleur fault?

I have a new specialised pitch pro mountain bike with sram gearing. After my first major off road event I have got problems with the gears. I am able to go down the gears (which is actually pulling the gearchang cable) but can't go back up the gears there does not seem to be enough spring tension on the rear deralleur to move the rear mech tried lubing pivot points etc but no better. Any body any idea or maybe come across this fault.

cheers Kev