I have been random breeding until I realised I only had 2 to go, then I started experimenting. I havent got far, but decided to come look for help. I was (and still am) missing the Leaf Frog, so I took your list and scrolled down to find how to make one.
You say...
Eggplant Frog + Spotted Leaf Frog = Leaf Frog
What really happened...
Eggplant Frog + Spotted Leaf Frog = Leopard Frog.
The only thing I could say is that perhaps the starting frog makes a difference? ie I started with the Spotted Leaf Frog, and bred it with the Eggplant frog, not the other way around.
*edit to add my failed attempts*
Here is my small list of failed attempts (I have barely got started!)
Tiger + bullseye = bullseye
hypno + bullseye = hypno
Tiger + hypno = Tiger frog
Whirlyflower + heart = heart
Tiger + leopard = Tiger
Sunflower + Surfer leaf = surfer leaf
Bullseye + heart = heart
Sunflower + Leopard = Sunflower
Leopard + Whirlyflower = whirlyflower