If you are using 4 sticks of memory it is running in dual channel. Dual channel is if you have it in slots 1 & 2 or 1, 2, 3, & 4. Some boards you can run in slots 3 & 4 too.
Any time you run 2 sticks or 4 sticks you running in dual channel as long as you have them in the right slots. Look at page 16 it will show you what slots will be dual channel. " Dual Channel Memory Configurations Table"
Why it is not using all the memory? It could be how you have the computer setup. You using on-board video? What OS are you using? If you are just using to play games and go on the internet any thing over 16 Gb you are really not using.
