My observations indicate that certain monsters are just more lucky then others at yielding rares and thus if you are breeding with one or two lucky monsters then your chance of getting a diamond goes up, but as they add more rares to the game your chance of getting a diamond goes down. Personally I think it is the monster's name that determines its luck, but it is only a theory nothing more. In my case I have an inferoo named Twig that yields an above average rate for rares (and was a parent for both my diamonds). The other parent for my two diamonds was a Eaguana named Sveta. So if you have any hybrid type monsters named twig (twig is a green monster name so any monster with a green element can have it) try throwing it into the mix. See if that helps.
My guide lines for lucky monsters:
1 in 2 monsters on average are rare - both parents are lucky (and if they have 4 elements amongst them then spam it until you get a diamond or something changes)
1 in 3 to 1 in 5 monsters on average are rare - one parent is lucky the other is average
1 in 6 to 1 in 8 monsters on average are rare - both parents are average or one is lucky and one unlucky
1 in 9 and lower - one or both parents are unlucky and you should probably stop using both of them in any future breeding.