So I've run into a bug recently: When I kill dragons, they just lie there without burning up. No soul is absorbed. I've gotten (and heard of) this kind of thing happening before, but as isolated incidents, often due to a dragon respawning in an already-cleared lair. This isn't like that, though: Regardless of whether I've encountered them before, I don't absorb dragon souls, period.
I've got a ton of souls from before the bug and I know how to add more of them with the developer console, so it's not a problem that impacts the strategy aspect of the game. Killing dragons just isn't the same without all the fire and chanting.
I run the game on Steam, have all three DLCs, and a few mods. None of the mods seem like they'd modify dragons, though.