I believe that the Radius Full Page Display card required a resistor to be cut on the logic board. It snapped on top of the 68000 chip and also there was another connection that had to be made with a spring. I remember trouble with this when I tried removing the card from my Plus.

My Plus also had 4MB of RAM, I seem to remember that somehow also required some hardware modification.

If you can, inspect the logic board for any cut resistors.