Also, after testing with Necrotic Plague + Unholy Blight - it doesn't use UB at all. UB should be used on CD and we should be using Festerblight rotation to maintain our NP @ 15 stacks until UB is available again at which time we use that to keep our NP @ 15 stacks and switch to normal rotation, and once UB has ended we go back to Festerblight rotation, rinse and repeat.
The only difference between normal ST rotation and Festerblight rotation is instead of using Scourge Strike on Death runes, use Festering Strike in order to extend the time that Necrotic Plague is at 15 stacks. The time of your Necrotic Plague should be extended long enough to match or exceed the time remaining on your Unholy Blight cooldown.
The above is taken from:
Son of a Lich | Acherus Forums ? View topic - Skullflower's Death Knight DPS Guide
Lastly - NP did fall off a couple of times. I had 97.8% uptime on NP in a LFR Brackenspore fight, there is no reason why it shouldn't be 100% uptime - I was always in range of the boss, I didn't die. I did switch to the big add when it spawned, but it had a 15 stack of NP from Brackenspore, so I have no idea why it would have let NP fall off.
EDIT: Also, DnD isn't being used when using Necrotic Plague talent, tested on multiple bosses in LFR with Specialkey enabled, not used once.