Eevee must have maximum happiness during the day then level up. you can make it happy bye battling and giving it the soothe bell. if you are in platinum you can take it to the ribbon sydicate to get a spa treatment, to get a massage in veilstone city and feed it poffins. in hg ss you can take it to pallet town to get one of daisy's beautiful massages. In both games feeding vitamens increases happienes. do not let it faint or use bitter medicine on it that will decrease happieness. If you evolve it during the night it will be umbreon so to get espeon you need to do it during the day anywhere between 5am-8am.