Hi Mate...Barry has nailed most of it above. Have you bought it SH as someone could have been tinkering with the reg set up and locked the valve down inside. I wouldn't expect that to be the problem though as i've only seen that once. Make sure the cylinder is full and properly screwed on, when cocking make sure you bring the cocking lever fully back so that you can hear a click as the stabiliser is engaged and then fully forward so that there is a positive 'clunk' when it locks closed.
What's the history of the rifle?