I put a Male Mimikyu and a Female Grimer-A in the daycare and the daycare lady keeps saying, "They don't seem to like playing together though." According to the Pokedex pages for Grimer and Mimikyu, it says they are both in the Amorphous Egg Group.
Just give it time. I've found that breeding in Pokemon S&M takes an absurdly long time, especially between Pokemon that are in the same egg group- heck, even the same species! Just keep running around, and you'll have your eggs eventually.
Best of luck!