Did you turn -showscripterrors in your startoptions on? Do you got Errormessages?
Did you create in the description.ext of your mission a "TaskSucceed" or "TaskAssigned" class?

Should look something like this:
class CfgNotifications
{
class TaskSucceed
{

		title = "Task has been succeeded";
		iconPicture = "a3\ui_f\data\map\Markers\Flags\nato_ca.paa";
		description = "%1"; //Text inserted which is given by the second statement
		color[] = {1,1,1,1}; //white color
		priority = 0;
		duration = 6;

};
}

["TaskSucceeded",["Disable the nuke"]] call BIS_fnc_showNotification; should now work.
Not tested though

@Heisenberg for the mentioned default templates that step is optional. However, with or without it the result is the same ;)

Edit: no scripterrors.

Alright.
Then the default template should have a problem, because with the cosumized one (eg the one is added) it works every time.

The description for task notifications is %2 e.g
["TaskSucceeded",["", "Disable the nuke"]] call BIS_fnc_showNotification;

Its like this so you can send your default task array to the notification function and it will display the title rather than the long description

@Larrow thanks! Issue solved. Hope BI can update the wiki, it's misleading ;)