If someone had pulled the breakaway pin, and left it out for an extended period, this could have damaged all the electro-magnets through over-heating. ( I am only quoting what I have read. I have no direct experience of this) I would try one replacement magnet on a wheel to see if that works. If you still have a problem I would install one complete back plate assembly with new shoes and magnet, so as to give you a firm basis for further step-by-step investigation. One possibility, and I have experienced this, is that the Dexter parts manual incorrectly gave the part number for Excella brake shoes, and I wasted many hours attempting to get the wrong shoes to operate.
Nick Crowhurst, Excella 25 1988, Dodge Ram 2500 Cummins Diesel. England in summer, USA in winter.
"The price of freedom is eternal maintenance."