When I try to talk to Serana, she talks but doesn't give me the option to tell her to follow me. I've already beaten the Dawnguard quest line and Serana is human now and I just had Lydia as a follower, if any of that helps. (I'm playing on the Xbox 360.)
From the Skyrim Wiki:
If told to wait, the option to "Follow me" may not appear when spoken to. If the cell is unloaded, she may disappear forever and it may only be fixed with console commands.
If the main questline for Dawnguard is finished, asking to part ways may fix this. If she will not continue following during the questline, she will appear at scenarios requiring her. If not, using ally commands to move her into the area may also work.
Also, sleeping for 3 days and then entering a new area will cause her to come running up to you.
Another Wiki has a similar solution:
- To get Serana following again after asking her to "Wait", go somewhere far away (like inside Fort Dawnguard) and wait three to five days.