I was going to say AI too. If you think she is ready and do not want to miss the breeding then I would do an AI for a few days then try natural again.
When we bred Siren we could not tell when she was ovulating even with tests done at the vets. We did AI's starting at the 11th day for one week and on the 17th day she ovulated and we got two natural ties. Based on the whelp date of the puppies she took with the natural breedings.
Have you ever bred this bitch before? if not she might ovulate late like Siren did. Also just because a bitch is flagging does not mean she is ready. Siren flagged the entire heat cycle but would not let a male lock up till she ovulated at 17 days.
If your males are not interested then I too would guess she is not ready but you can still do a few AI's just in case. Even if the male is not sexually excited by the female if you know what your doing you can still collect from the male with the right moves.
Light a candle put some slow music on and collect your male