With the fiance visa, the number one cause for denial is we see is the Affidavit of Support or proof of income. Income scrutiny is under the microscope since “extreme vetting” became the standard. These are out of concern that the applicant is possibly going to become a public charge and collect means-tested benefits. To avoid this, you should more than meet the income requirement by yourself, and ideally earn 20% more than the income requirement, for at least three years in a row. You’ll want to show tax transcripts, W2s, bank statements, etc. There are increasing RFEs demanding applicants who even meet the income requirement to still find a joint sponsor who earns more, although some countries, such as the Philippines, don’t allow joint sponsors for fiance visas.
Another common reason for denial is not providing adequate proof that the applicant intends to marry within 90 days or they haven’t convinced the officer that they have a bonafide, legitimate relationship. For this reason, you need to build up your bona fides, such as getting an engagement ring, planning an engagement party, showing pictures together, detailed wedding plans, etc.