Turn F4 off. And F5 on. I've never had $c0 memory on and I get the full amount (and also working with either physical trapdoor slowmem card or virtual ACA chipmem for the same physical card).
I doubt this will fail. But if it does, well, I've experienced once that I've had to press D, after having installed and removed the Kipper2k card (while ACA500 inserted).
After that, set your settings like in the screenshot, but turn F4 off and as Jens says F5 on.
Then press S and enter.
You didn't have a CF card inserted so it's not for you, but for those who get less memory than expected only when the card is in, check the total partition-buffers by typing addbuffers dh0: 0, addbuffers dh1: 0, and so on.