You must be sure its happiness is high enough, and after that, you must level it once, for evolution to actually occur. Pokerus won't do anything happiness wise except double EVs.
Also be sure you are levelling it during the correct times in night time, since they work differently in Generation V because of seasons:
"Seasons affect the time of day, which now varies depending on the current season. During spring, morning begins at 5:00 AM and night starts at 8:00 PM ; during summer, morning begins as early as 4:00 AM and day lasts until 9:00 PM ; during autumn, morning begins at 6:00 AM and night starts at 8:00 PM ; and during winter, morning does not begin until 7:00 AM and night starts at 7:00 PM. The day is now divided in four different parts: morning, when floodlights in the cities switch off and the sun starts to rise; daytime, when everything is lightened; evening, when the floodlights switch on in the cities and everything starts to be lit with an orange hue; and then nighttime, which is characterized by a darker landscape, both in the overworld map and in battle." - Bulbapedia
You can also use this chart made from Bulbapedia for gaining amounts of happiness.
But if you are absolutely sure the happiness is high enough, simply level it some more, its bound to evolve.