Well, the Bond flicks don't always explain what happened between one Bond girl to the next. Felicity and Austin were not married, so they could've just parted ways.
Since the first movie was not expected to have a sequel, they just let Vanessa and Austin settle down. But, when the sequel came, they had to have a new girl (it's the Powers way), but at the same time explain Vanessa's disappearance.
For Felicity, this just wasn't necessary.