User blog:Catman3/Breeding Poo

I've looked at the breeding calculator for Poo and tried the combination that supposedly has a 66% chance more than 15 times. I've gotten many mud dragons, dark, hedgehog, and even plankton dragon in my quest. My question is: Does breeding certain dragons get harder the higher level you get, or is it all based on difficulty? I'm level 50 now, and I have bred level 7 pirate with 7 wizard over 10 times, which should have a high chance for giving poo. The same with 7 petroleum and 4 colossal, which I have done at least 8 times. Does anyone know what I am doing wrong? I have been trying to breed a poo for 2 weeks, usually with both breeding grounds at once.