Then you had an A500 with 2 Meg Chip!
For 2 megs of Chip, you need an 8372A/B (different to an A or a B) or an 8375. The 8372A/B is the same core as the 8372A, but has a slightly different pin-out to give it an extra address line. I'm not sure what (if any) differences there were with an 8375.
500 (rather than 500+) motherboards simply aren't laid out for 2MB of chip RAM. In order to get it, the extra RAM needs to be on a small board plugged into the Agnus socket, along with an 8372A/B or 8375. There's no other way of doing it - the chips have different pinouts and the lines just aren't present on an A500.
Slightly strangely, my A3500 has an Agnus that's stamped 8372A and has had the /B drawn in by hand. Early part, I guess.