have you checked for leaks, a very small leak anywhere an it won't carbonate, a Starsan solution (sanitizer) put into a spray-bottle is great for finding leaks, it'll bubble like soapey water does and you can see the smallest of leaks, if you find a leak, use 3-4 wraps of teflon tape on threads. i'd disconnect the line to the keg an turn pressue up around 30psi to check for leaks. if there are no leaks at the regulator or lines, then i'd check the the keg for leaks. you need to use keg-lube on all rubber parts to insure a good seal.