I think I have a bug: just for kicks I decided to NOT start the main quest (fetching the dragon stone for Farengar) and started (and completed) the Dawnguard expansion. Now I am unable to absorb any dragon souls after starting the main quest in order to explore the Dragonborn expansion. None of the five I have killed (initially at the west guard tower, and at several word walls since then) they fight me, die and then just sit there. I can loot the bodies but not receive any souls.

I suppose that I can use the console to add souls artificially, but I strongly suspect that killing Durnehviir in the Soul Cairn - without being identified as a Dragonborn (except by Durnehviir himself) - has caused the glitch. Anyone else happen to take this path and experience the same thing?

Using the console (Player.modav dragonsouls 1) did NOT jump start the process. Guess I will have to toggle off mods one after the other

I've done Dawnguard before the MQ a couple of times. Never had that bug though.
Maybe a mod has caused it.

The only time I know of a dragon not giving a soul, is when you have cleared a dragon from the wall, and then go back for another dragon has spawned. Hope that made sense.