ESTA Issues

Hi all,

I'm helping my girlfriend apply for a VISA to visit the USA next year (from Japan) with the following details:

- I am American, she is Japanese. We are not engaged, nor does either of us hold dual citizenship.

- GF has been to USA thrice. Twice for travel, once for work, no visa issues.

- I-94 is fine, all exits were legal and on time with proper documentation.

- All the ESTA questions that would disqualify someone were answered as no, and GF has no criminal history in either country. All other fields were filled out with proper information.

- I am hosting her at my house in America, as well as paying for her travel and living expenses while present in the USA. All of this info was listed on ESTA proper as well.

- Backup info was for my parents who also live in said house that was listed as the place where she would stay.

ESTA auth was denied - customs said it was a background check issue, but there's nothing that could have been flagged by it, so now we're left wondering what it could be.

Does anyone else experience with this type of weird no reason denial? I'm not sure whether I should move ahead with another ESTA or B-2 in this case - there are 3 months until the travel date, so I have time, but I'm wondering how much this failed ESTA attempt hurt our chances of approval for B-2, despite the fact she should be perfectly fine to go.

Thanks!