Breeding

I am confused about breeding. Finding two different gender, level 15or higher monsters, and of the same element it is not enough what am I missing?

[Koenaj]

They need to be of the same evolution branch.
You can't breed a Draconius with an Ignightmare, but you can breed a Draconius with another Draconius (opposite genders ofcourse), or a Draconius with a Draco.