It could be the cocking rod being too long (not screwed in all the way) like skeet said, or it could be the nut on the end of the cocking rod being unscrewed or loose... Check both!
De-gas, take the hopper off, pull the back block back by hand whilst looking down the feed tube and see where the bolt stops, normally you would hear the "click" of the gun cocking just as the bolt is about to go out of sight....
The bolt SHOULD go out of sight, even if the marker doesnt cock.
If this is the case, and your cocking rod is most definately screwed in all the way,then your sear lug might be too short (might have worked its way loose, or worn away abit as it got "broken in").
It is normal for new mechanical cockers to suffer from timing changes as they work their way through the first few thousand shots... The thing is, to fix it you have to alter the markers timing, and the chance is there to really screw things up! (mind the punn)
I think i'll stop there and let you check all that stuff before going in to the intricacies of timing a cocker!