It takes time for happiness to accumulate. Perhaps you need to wait longer. It needs to reach 250 Happiness before evolution, so you probably have to wander around more.
- Let Eevee hold the Soothe Bell, which increases the rate of happiness
- Remember that it needs to level up to evolve.
- Use EV lowering berries, such as Tamato Berry, Qualot Berry, and Kelspy Berry.
- Remember NOT to put it into the PC or daycare.