Two RAM Slots (1 & 2) won't recognize RAM?

I've got six 1MB simms and have tested them all in pairs on two different SEs. On one machine they all grant the machine the full 4MB, on another, on 2MB no matter what combination of 2x2 that I do.

So I'm thinking something is wrong with the two RAM slots, but I no way of telling why. Are there diagnostic software tools for System 6/7 that might help? It is feasible to donate ram slots from another board? (Not sure if I'm up for that)

I did not know that there was a physical jumper on the boards. Removing that fixed the issue. The 800k board I have has a resistor soldered to it but it has always recognized 4MB.