What triggers combos in Plague Inc. Evolved?

Plague Inc. Evolved has different combos of symptoms and abilities. E.g. if you get Liquefactive Necrosis and Autothysis with the Necroa Virus, you get the Boomer Combo. This should have trigger a pop-up, a certain bonus and an achievement if it happens for the first time.

The theory sounds easy, but it is quite unreliable. Sometimes the combo triggers, sometimes it doesn't. And I have no idea why. Some of the achievements are still locked although I tried the relevant combos several times without success.

What exactly do I need to do to trigger a certain combo besides evolving the appropriate symptoms and abilitis? Is time or just a random factor involved here?

The combo doesn't always trigger when you have the required symptoms, its random chance, you have to cross your fingers if you want to get all the combos.