It couold be that the vlave dwell time setting is too short... ie the amount of time that the bolt stays back to allow a ball to drop into the breach. If this is the case check your owners manual to see which dip switches control this setting and increase it.
However, it is more likely that you are using ****e paint...
Is paint bursting in the barrel- yes/no?
Does the paint fit in the barrel - yes/no?
Is the paint dry- yes/no?
Is the paint bursting in the breach- yes/no?
Don't change the valve settings until you've answered these questions!