Yes, not enough evidence of a valid relationship is usually the biggest reason for denial. You usually must convince the interviewing officer that you are not committing visa fraud by faking a relationship. The relationship evidence may be a lot tougher for a K-1 than for a K-3 or CR-1.
A lack of minimum steady income for the petitioner is another reason for denial. They prefer to see a steady income from full-time permanent employment.
Though not as common, problems with her medical exam can result in denial.
Another reason would be a prior marriage without proof of legal termination.
It is imperative to keep good records of correspondence, communication, physical visits, photographs, etc., for later presentation at the visa interview. Which country she is from is a big factor in determining how tough they are on the evidence requirements.
Keep in mind that many if not most denials are not final, but may be reversed with submission of further evidence, correction of medical problems, etc.
Ray
