You mean that the lines that are supposed to be said by the NPC are never said and you only go from PC reply to PC reply, right?
The only thing I can think of that you could try, is to add the tag of your NPC to the "Speaker" field when the NPC is supposed to be talking.
How do you start the dialog by the way? With a script or by "talking" to the NPC?
Also, as a sidenote, you can copy and paste entries/replies in the DLG editor. The copied entries will then show up as "Text (already listed)" and everything that comes after this line won't be listed again. That saves time and space when one branch of the dialog is accessible from several points in the dialog. Also, if you then edit the original, the changes will be applied to the copy as well. And if you knew all of that, just pretend I never wrote it