Ok, I suspect what you're seeing is not SQL not using the other processors, but a side effect of how affinity is set on higher processors. Your observation that all processors are ~50% under load suggests that as well.
Check sys.dm_os_schedulers, and look at the load factor, the worker count and other columns. That'll tell you if SQL is using those processors

Keep in mind that overall processor usage will not all reach 100%, it's very, very hard to max out modern processors, and you could well have waits or other resource constraints that's limiting throughput.