Skyrim is a very buggy game.

Psst her name is, Serana.

I've been playing A LOT of skyrim lately, but haven't really done many dawnguard quests as of yet. You'll find on steam forums or elder scroll forums that a lot of other people are having similar issues. Heck I can't even finish a siege on Whiterun 'cause my dude Ralof is bugged, but on forums I found that I have to complete the main quest for him to be unbugged - since he's required in a main quest mission. Sad face. You may have that issue too, who knows. I'd do some research up on it and see if you can find solution.