There are a few reasons why that could be happening. The screw could be too long and is hitting a more rigid material it can't penetrate behind the piece of lumber you are fastening. Or you may be fastening into a knot in the wood. Knots in wood are like scar tissue and are much tougher to drill through. Drilling into a knot can quickly stop a wood screw, or even break it if you're not careful. If you need help with your application, contact our support line at 866-573-0445 or email us through our contact form