Check the air tank to determine if it is full. If the tank does not have enough air left, the gun will not be able to re-cock after it has been fired.
Disassemble your gun and oil the internals parts. Use paintball marker oil rather than some other type of oil.
Examine the o-rings throughout the gun. If any of them appear to be worn or broken, replace them with new ones.
Reassemble the paintball gun and attach the air tank. Test the gun to see if the bolt re-cocks after it has been fired.