It sounds like the board needs them in pairs. This is normal on many boards. This is sounding like a problem I had with a Tyan server board. You had to install the memory in a certain sequence. Slot 1 and 3 first, then boot, then shutdown and fill slots 2 and 4. Any other way and it would either not post or the OS would only see half the ram.
Troubleshooting these types of problems is never easy.