The success of breeding depends on several factors, the primary being rarity of the dragons in question. Rare dragons are far less likely to produce an egg, though pairing two rares versus one rare and one common has an equal chance of producing. In some cases, the pair may refuse, producing no egg and disallowing the pairing of those two dragons in the future.
Bred eggs, unlike the eggs taken from the biomes, can be abandoned immediately, cutting out on the wait time of 5 hours. If an egg is bred but the user has surpassed their scroll limit, the egg is auto-abandoned.
BSAs affecting breeding
The following dragons are hybrids: they can only be obtained through breeding and cannot be found in the cave:
The Geode Dragon can only be made by pairing the following:
The Hellhorse Dragon can only be made by pairing the following:
The Shallow Water Dragon can only be made from the following:
The Soulpeace Dragon can only be made from the following:
Striped Dragons (different colored versions)
The different colored versions of the white striped dragon can only be obtained by pairing the following:
The Two-Finned Bluna can only be made from the following:
The Ultraviolet Dragon can only be made by pairing the following:
Though many dragons are free to be paired with any other dragon, some morphology types are restricted to breeding with themselves. The other breeding groups are Drakes, Two-Headed Dragons, and Pygmy dragons.
Message received and explanation
"The dragons refuse to even go near each other."
This means that the two dragons you tried to breed will never breed with each other. It can happen between dragons of any rarity. Breeding Holiday x Holiday or Retired x Holiday any time besides at least one of the Holiday's breeding season will result in a refusal. (Ex: Christmas x Halloween during the Valentine's breeding season, Frill x Valentine at anytime besides the Valentine's breeding season)
Currently, once a pair of dragons has refused, the refused partner is moved to the bottom of the list of possible mates and is not available to attempt breeding in the future.
"The dragons don't really show much interest in each other."
This means that the two dragons don't want to breed at the time but are capable of doing so in a future attempt. Once a pair of dragons had no interest, the partner is moved to the top of the list of possible mates.
"You attempt to breed the two dragons, but no egg is produced."
This means that the dragons bred successfully but no egg was produced. Once a pair of dragons bred no eggs, the partner is moved to the top of the list of possible mates.
"The dragons breed and produce an egg."
This means they have successfully produced an egg. Once a pair of dragons bred an egg, the partner is moved to the top of the list of possible mates.
In Halloween of 2013, a user experienced a multiclutch breeding glitch by pairing a Guardian of Nature with a Black Marrow, resulting in 4 Guardian of Nature eggs. It is unknown how this issue was resolved.