Is your dialogue quest "start game enabled"? If so then welcome to the club mate

There was a bug introduced in 1.7 that is a bit random, but the general consensus is that start game enabled is broken. A quick test to see if this is the case is load the game with your mod enabled, save, then reload that save. If your dialogue magically appears then its confirmed.
It seems the best way around it for the time being is to instead start your quest via script somewhere (using myQuest.http://www.creationkit.com/Start_-_Quest). It's really up to you how best to do it for your mod. You could attach a script to a triggerbox and use an OnTrigger() event, on a book/note base object using http://www.creationkit.com/OnRead_-_ObjectReference, or on a button activator using http://www.creationkit.com/OnActivate_-_ObjectReference. Some are even getting away with having a quest which only contains that script, which for some reason the bug seems to ignore

The following script should work. you'll need to change "OnWhatever()" to the event that that you've chosen. If you want to go with the starter quest method then you'll need to extend "Quest" rather then "ObjectReference" with an http://www.creationkit.com/OnInit event I think.
Scriptname QuestStartScript extends ObjectReference Quest Property myQuest AutoBool doOnce = false AutoEvent OnWhatever() If doOnce == false myQuest.Start() EndIfEndEvent
When you add the script don't forget to click on the properties button afterwords and fill the myQuest property with your quest
Good Luck
- Hypno