One of the common complaints against Plague Inc. is that a user apparently wins a game but then the next level does not unlock. Plague Inc. might be a tad glitchy at times but this is almost always because the user did not in fact kill everyone. Plague is not about killing 6.5 billion, its about killing EVERYONE.
So how does this happen? A game ends and you look at your chart, and it's clear that everyone is dead right? It probably looks something like this:
In fact, when this graph was taken, 500,000 people remained alive. The graphs are interesting but can be misleading.
The problem is, you did NOT in fact kill everyone. This happens when your disease is too deadly to be actively transmitted throughout a country. If you have not received notification that "There are no more healthy people in the world" and the game ends, you have lost. The most likely culprit is probably Greenland or Iceland. Pathogens can be extremely difficult to transmit throughout the entire populations.
If you find yourself in a situation like this you need to enhance your pathogens ability in cold climates, that's the "snow flake" to level 2. That'll get those pesky Greenlandians! Otherwise, your only hope is necrosis.