There is a bug with Serena when you tell her to "wait" and then try to get her to "follow" you again the option to follow is not there! She just stays in one place untill you tell her to leave, at that point she starts to leave but you cant get her to follow you again ever!
I do know the place where the bug starts however. It starts when you try to go back to the soul carin after completing the dawnguards questine ( reguardless if you have her with you or not) to tell her mother that we won and to use the soul carin. The moment you enter the castle (on the other side, the one that leads to the soul carin) is when the bug takes hold of her and you cant tell her to wait and then follow again!
Beathesda, please fix this!
I can confirm this. It was nice having her for a follower while it lasted. She had great dialogue and commentary on her surroundings, but now she's like any other NPC who won't follow you, and the only dialogue is asking her to "blood some of your arrows."
I'd like to add to this. Can you confirm that her carry weight is extremely high, as in, higher than it should be? Because I returned to my former follower (Jenassa from Whiterun) after Serana's meltdown only to discover that her carry weight is a lot less than before.
Vampire Serana I had loaded with 5 staffs, 6 dragon scales, 3 or 4 dragon bones, a couple of greatswords, scaled armor suit helm and guantlets, vampire armor, an ebony bow... That should be overburdening, right?
Jenassa now... A suit of light armor, a dagger, an ebony sword, a bow and after that room for maybe 30 pounds more. What the heck? I know her carry weight has been reduced a lot... by Serana or some aspect of Dawnguard. I don't know how.