Porter cable makes a number of brad
nailers (18 gauge) that very quickly suffer from the problem that you are having. You did not mention the model number of your gun but if it is one of these, (bn125a, bn200a, ns100a) kit # 60061 will fix the problem. It costs between 25 and 30 dollars.
Google that part number or order from
www.dewalt.com.
Dewalt owns the porter cable brand. Before you order the kit, check that the driver blade is not bent and stuck. The driver should travel freely down the cylinder. When you install parts, use Danco silicone grease to lube (availabel at Lowes in the plumbing department) If your model # is different from those mentioned above, please email me and I will
recommend a repair.