What's your load average? What is the box mainly running (Oracle, apache, sendmail)? If your load average, N, is always less than 12, then there's no reason for the OS to use N/12 of each processor when it can just fill N processors and leave the others idle. Also check that your application is actually capable of doing 12 things at once (i.e. that you haven't configured apache to use only 6 threads, or whatever).

Peter