I'm playing through the Dawnguard faction questline, and I'm at a part where I have to find a Moth Priest. It tells me to ask around Dragon Bridge for any hints, but no one knows anything. They keep telling me to go ask the guards. When I do ask them, though, they don't say anything apart from their usual banter.

Why is this happening? Also, if it helps, I played through the Stormcloak questline and had the Imperial guards get replaced by Stormcloak Soldiers.

Talk to the kid or follow the road across the bridge and look for the overturned carriage.