I'm trying to get an Adamant Heracross for my competitive team, but to get that I need an Adamant Pokemon with synchronize and adamant nature. So I used Abra. Unfortunately I had no adamant Abra, so I needed to breed one. I had an Adamant trevenant, who is in the grass and amorphous egg group, so I gave it an everstone and bred it with a cacnea, who is in the grass and human egg group. Once I had my adamant cacnea, I gave IT an everstone and TRIED to breed it with one of my Abra, but it didn't work. They're both in the human egg group. They're different genders, but they'd rather play with other Pokemon than each other...
What am I doing wrong, trevenant bred with cacnea, so why can't Abra. Is it because cacnea's only has human as a secondary egg group? I just don't get it!