First of all, you must disassemble your paintball marker as the instruction manual tells you to. Usually, there are only 2-4 screws that need to be taken out in order to disassemble the gun. You will have to pull the inner workings of the gun from the chamber.
Next, take a closer look at the gun internals and the bolt for signs of wear, scratches or nicks. It is recommended that you should wipe down the internals thoroughly by using a cloth dampened with gun oil. If you notice any scratches or wear, you will probably have to replace the internal components or ask for help from a paintball shop to point you in the right direction.
Moving on, you should replace all the O-rings from your marker's internals, along with the O-ring on the CO2 tank and gun barrel. You should know that the vast majority of problems that occur with paintball guns arise from a gas leak somewhere in the system, and these O-rings are your first defense against a potential leak. It is advisable to clean the gun barrel and the marker chamber thoroughly by using a rag dampened in oil.
Your next step would be to replace all the springs inside the gun. These springs will determine the velocity of your paintball gun, so if by any chance you have noticed a drop in downrage velocity or distance, replacing the springs might fix this issue.
You will now have to reassemble the paintball gun by simply reversing the disassembly steps and test-fire it for several times. Do not forget to check all of the gauges and lines on the paintball system and verify that the tank is fully charged. Now, you gun should be ready for action.
If you still encounter problems with your paintball gun, it is highly recommended that you should take it to a professional paintball gun service or a paintball shop as they will be able to find out the root of your problem.
In the end, here are the most common problems that occur when using a paintball gun: the paint is too old, the velocity could be set too high, low air pressure in the tank, the ball detent may be dysfunctional or you could have dirt or debris inside the gun's barrel. Needless to say, the paintball gun barrel should be kept clean and clear of debris, and you should as often as you can fresh paintballs. Also, acquire a squeegee which will help you clean the paintball gun on and/or off the field. To use it, simply thread it through the gun's barrel, pulling from the other end.