Could be several things.
The most common reason for the problem you're describing is that the fine velocity adjustment is "between clicks". Take the barrel off the marker, and uncock it. Using the proper size hex key wrench, turn the velocity adjuster inside the upper bolt until you feel it click into one of its indexed positions. This causes the two ball bearings to retract inside the upper bolt. When the adjuster is between positions, it will cause the ball bearings to push outwards, causing excessive drag, which causes the marker not to re-cock.
The oring on the striker may be worn.
Make sure the upper and lower assemblies are properly lubricated.
Make sure your main valve body is set to use the largest air transfer hole. Take out the upper bolt and striker, and look down the lower chamber to the valve body. You should see that it is marked with an indentation and some red dye. Make sure this mark is in the 12 o'clock position.
If you continue to have problems, call 1800.7-AIRGUN and ask for Steve in paintball.