Forum Thread
Daycare trouble
Forum-Index → Help → Answered → Daycare trouble1) Different Original Trainers. You can check it in the Pokémon's profile
2) Exactly same species. For example, pairing 2 different gender Gastly gives you more chance to get eggs than a pair of Gastly and Haunter/Gengar
3) Different genders, of couse
Based on your description, it seems that you're pairing a Pokémon with a different species, or its evolution. Moreover, their Original Trainer is the same. You should check those two if they've met the requirements above.
If those two are true, I suggest getting the Pokémon that has different Original Trainer, and is the same species as the other one.