Some elements cannot be mixed together directly and this is intended.

Two different scenarios here:
1) The Dragons appear in the list of breedeable Dragons BUT they are INCOMPATIBLE.
This ONLY happens with Dragons that have just 1 Element and there are only some specific couples, e.g., Flame + Ice, Terra + Metal, Light + Dark.

2) The Dragons do NOT appear in the List of breedeable Dragons. This ONLY happens with the Dragons that have one Element coming from the Ancient World. (Dragons with Ancient World Elements CANNOT breed!).

So remember, Dragon Master, that you will have to use hybrids Dragons, meaning Dragons with more than one element and with NO Ancient World elements! :)