Memory slot went bye-bye

I'm new to macs, I just Installed more memory on my G3 B\W 350 sportin' OS 9.2, on Intial fire up It found all 512MB 128 per slot turned It on Later I get a internal mem check error It finishes loading but when I checked the memory It reads 384MB and the 4th slot Isn't there.
I've tried swapping the sticks of mem around and still no luck have I missed somthing??

 

It's back

Well good news It found the fourth slot again, It dawned on me after a unproductive 4 days tring to help my brother breath life back into an older system, pulling and tring different modems...anyway i thought I'd try reseating the memory stick a few more times....sure enough the system found It on the first try.
It's been awhile since I had a computer be so particular about this kind of thing, so the lesson here when In doubt pull It and reseat it Into the slot ya might get lucky.