DMA Memory Mapping

I have an old ISA A/D data acquisition card that utilizes DMA channels 1 or 3. I recently tried to increase the amount of system memory by 256MB SDRAM. The card quit working and the software configuration interface claimed the selected DMA channel 3 was invalid. So I tried mixing and matching the memory I had on hand and even canibalized another system to put together three 128MB SDRAM boards to total over 256MB and the DMA failed again. The DMA would work with less than 256MB but not more than 256MB.

Is this about Memory being mapped differently based on the available amount?