Nail gun driver doesn't return after firing
Make sure the supply air pressure is within the pressure range specified by the manufacturer. If the supply pressure is too low, there may not be enough pressure built up to return the piston to the firing position. 

Also make sure that all the vents are clear, and unobstructed.  If the vents are blocked, it could increase the resistance on the returning piston.  This added resistance, can prevent the piston from returning to the firing position.