How to make memory ram slot work?

Hi guys, Im just the new member on this site.

I have a question that keep in my mind year ago. I got iMac

late 2009 27inch and obviously as we know it comes

with 4 slots of memory ram. However, I can't use the other 2 slots at the back

(not the front).

I thought it may get stuck of dusts in side the slot, I tried to do different ways

but can't make it work.

I want to upgrade more ram in other slots left coz I still got two more spare rams.

Please give me some advices guys Cheer