So..not sure if its a glitch or not but my favorite vampire Serana will no longer folliw me..I told her to wait in a fortress I was infiltrating and she went back to Fort Dawngard..when I arrived at the fort and attempted to get her to folliw me again but the topic wont come up..any ideas?

PC? Try set playerfollowercount to 0, It Clears followers and allows you to recruit again.
Or click on her and type "disable" followed by "enable" or try "resetai", or "recycleactor" if these didn't work.
You can try this aswell,
Click on her and type setessential 0, then kill, then resurrect, then setessential 1. type help serana to get the ID.
I'm not sure if any of these will work, just trowing ideas out there

Serana is pretty damn buggy at times. If you tell her to wait during certain parts of the Dawnguard quest it can screw her up as a follower. If you're on the PC you can do the above, but if you are on the console version then all you can do is reload a previous save file =s