I am using Ultra Moon and I am trying to get an Eevee using my Espeon (M) and I tried breeding him with my Ditto. I was told they didn't get along well. So I switched the Ditto out for a Fomantis. It didn't work either. So I tried an Audino. And I found out that it didn't work because the Audino was male. So I tried a Mimikyu. And that didn't work either. I did some research on the site and I found out Fomantis and Mimikyu didn't work because they were in seperate Egg Groups. But I only want to know why Espeon didn't breed with Ditto.
Now that I know exactly what you're asking I can answer this. Pokemon that are a different species and have the same trainer will only have a 20% chance of producing an egg (without the oval charm), which is why the game says they don't like each-other much.
If they don't get a long well, they will still breed. It will just take longer. The egg has a 19.8% chance of being made every 256 steps you take, so just keep walking!