Upgraded Mac mini issues

I started with a 1.5 core solo , 512 mb ram , 60 gb hd running tiger. Now it's a core2duo (t7200), with 2 gb of matched 1 gb pc2 5300 667 ddr2 Hynix ram, and a 250 gb hd running snow leopard. Which by the numbers seems to be a decent little machine but now to the problem.. I only have one ram slot visible anymore, I have even tried going back to the original ram and it still shows only one slot active. I'm not sure if before the upgrading I had both slots because it was such a small amount I never really looked at the info. There is no signs of damage to the slot or to the board .. Any help would be immensely appreciated.

What does Apple System Profiler (About this Mac) have to say under Memory?

It says one gb if I have the two 1 gb in ,it says 256 mb if I have the two 256 mb in

Original specs from MacTracker for the 1.5GHz model state only one memory slot with 1GB maximum memory size so I guess that is where your problem exists unable to recognize second slot.

It will not fully recognize the 2 256 mb and the specs I find are that the max is 2 gb split @ everymac.com