Well, you're probably doing something wrong with your eevee. Just check your Eevee if they have met the requirements for it to evolve into Slyveon.
Requirements for Eevee > Slyveon:
- Have 2 or more hearts of affection in Poke Amie
- Have The Eevee with at least 1 Fairy Type Move
Make sure that they don't have a fairy type move while it has 2 or more hearts of affection, this will trigger it to make Eevee turn into Slyveon. Affection is commonly mistaken as happiness. These will be the requirements for the Eevee to turn into Umbreon.
Requirements for Eevee > Umbreon:
- Level it up at Night Time in-game
- Raise the Eevee's happiness to max happiness.
So, make sure you have met all of the requirements for Eevee to turn into Umbreon and try to be away from the requirements for Eevee to turn into Slyveon. For more information, try going to the Eevee's page and see how Eevee turns into Umbreon. If you want the fairy move, just try to be away from Poke Amie and get it to max affection once you evolve it into Umbreon.