I learned this trick in another thread:
I too was choosing flirt options with Lana. When the mission "Relax in the Cantina" came around, there were no flirt options available to talk with her. Only normal choices to talk with either Theron, Koth or Lana.
I restarted the chapter, and this time I chose every flirt option available when talking with Theron. Then when doing the Cantina mission, I now had the choice to normally talk with one of them, or [flirt] talk with one of them. From there, I was able to flirt with Lana, and lock in the romance.