samoht, are you trying to make her your girlfriend again after you have broken up with her? If that's the case then it
may (don't have to) be a bug that has occured to yours truly and some others that makes it impossible to declare someone your girlfriend/boyfriend/BFF/fiancé/fiancée/wife/husband if either of the sims have been any of those ever before.
If you are having problem with it, that could be that problem.