NFORCE6M-A Memory slots.

Hmm so i recently found out the 4th slot in this motherboard was not working. So I now have 1.5gb only. Last night i wanted to see certain things run under 1gb by taking the 3rd slot off. When i decided to put it back on it no longer worked. I tried to mess with it by putting the 3 sticks in different placing (say for ex. 1-2-3, 2-3-1, 2-1-3, etc). Anyone else come out with a problem like this? If any other info would be helpful just ask as a reply b/c I have no idea what would be needed besides mobo (in title). Thanks in advance.