Oracle 11.2 using too few memory
Oracle doesn't pre-page the SGA by default, meaning that when you start the DB instance, it will not initially grab all the memory until it needs it.  

See http://docs.oracle.com/cd/E11882_01/server.112/e40402/initparams201.htm#REFRN10174.  

I'd try this:


Set memory_target = memory_max_target = 24GB.
Get rid of sga_max_size, sga_target, pga_aggregate_target in your init parameters, unless specifically needed by some crazy vendor app (in other words, let 11g AMM do its thing instead).
Also clear the db_cache_size from your pfile.  You were basically limiting your cache to 5.4 GB.
Bounce the instance.
Beat it up with workload for a few hours/days (SQL queries, gather stats, rebuild indexes, RMAN backups, expdp).
Check in Task Manager for memory utilization.