Zydec A500 1.5MB

[psxphill]

Under KS 1.3, Amiga won't boot. It just flashes red ...
Under KS 3.1, it boots into WB, but it reports only 0.5 MB of additional RAM.

If it's like other similar boards then the first 512k is at 0x00C0 0000, the second at 0x00C8 0000 and the third is at 0x00D0 0000.
 
It's possible that the cable that goes from the gary board to the ram board has a problem.
 
What revision is your amiga's motherboard?

[psxphill]

It's rev 6a.

Make sure it's not set for 1mb chip ram from the trapdoor or on the motherboard. That could affect it.

[psxphill]

No modds of any kind. Both amigas have rev 6a mbos.

Are you using the same power suppy?
 
Other than that, it could be anything. You could try swapping chips.
 
I'd probably swap the cases, so the one that works is in the nice case.