where is it showing you only have 12gb available? can you show us some screen shots - upload them to an image sharing website and show links here.
what sticks do you have?
recent memory issues? what were these?
Is your ram all from the same kit?
If not, you may have compatibility issues.
What is the make/model of your ram sticks?
What is the make/model of your motherboard and cpu?
If you have 4 sticks of 4gb each, possibly one of the sticks is faulty.