Hi, my eevee is full felicity and it is sunny day and doesnt evolve to espeon. I dont know if it is a bug.

Your Eevee will only try to evolve into Espeon upon reaching the max amount of happiness. So you would need to faint it in a wild battle to lose some happiness first and then get it to the max happiness during daytime (10:00 - 20:00 ingame time) again. Happiness evolutions dont require a level-up for the evolution, its only about reaching the max happiness.

This is indeed a bug, first you'll want to:
1. Faint your Eevee (To lower its happiness.)
2. You fight more wild Pokemon max out your happiness again.
3. After you have 255 happiness again. You level it up during day, and you'll magically have an Espeon!

(Actually don't know if the level up is needed. But I'd say you should try it anyways)

Hope this helps.

Edited November 5, 20223 yr by Bfgo
Just adding a clarification that something might've not been needed.