If it seems to be from the nose piece , there are 2 main possibilities .
1 The plunger that drives the staple is projecting beyond the end of the nose piece when driving the staple . This will tend to look like a narrow straight line .Reducing air pressure may help .
2 The stapler is " bouncing " when you staple .
This will tend to look more like a dent or gouge , and if you hold the stapler to the dent, it will look like it "fits" . If this is the case, hold the stapler more firmly against the wood .This might require a hand on top of the piston area, and is more prevalent with higher air pressure .
Have you tried a test using the actual material you are going to be stapling ? Depending on the top material , may turn out not to be an issue .