Eggplant should ferment.

Maybe it's the combination of the miso with the salt? Miso is already very salty.
Plus aubergine/eggplant can be very moist already.

Maybe it's ratio of liquid to salt. The salt would draw the water from the eggplant.
The amount of water content in the eggplant would depend on the size I guess.

I grew baby aubergine last year and they were great. Not at all like the large ones. They weren't bitter and they had very little moisture in them. They were crisp.

Also came across this regarding the temperature.
http://www.culturesforhealth.com/fer...shooting-guide
Troubleshooting - down the bottom of page.
"Vegetables Don’t Seem to Be Doing Anything"

There's a section in a book below here that mentions starter cultures and the difference seasonally but it is about one particular type of eggplant - Almagro Eggplant

Molecular Techniques in the Microbial Ecology of Fermented Foods
By Luca Cocolin, Danilo Ercolini
Page 152

They mention addition of homemade (not bought) whey in this
http://www.westonaprice.org/health-t...-fermentation/
Lacto-Fermentation

Whey again and lactic acid brine mentioned.
http://thefermentary.blogspot.com.au...-eggplant.html