There are two likely culprits: grit in the cap threads, or a twisted O-ring.
In certain conditions, particularly in sandy environments, fine grit can get into the threads of the cap, which will keep it from sealing completely when you screw it on. Check the threads for any dirt or grit, and give it a thorough scrub.
Next, unscrew the cap and take a look at your O-ring. The O-ring is the black rubber ring inside the top of the cap—it looks just like a black rubber band. If the ring is twisted or pulled out of shape, your lid won’t seal correctly. You can usually pop the ring back into place, but if it’s permanently warped or broken, you’ll need to contact our Warranty Department for a replacement.