Most common and serious reasons? Not disclosing previous visa applications or stay in the U.S.; petitioner's status; civil or marital status, admission of charges all of which could have been prevented or waived.
WHY VISA APPLICATIONS ARE REFUSED
Having an approved immigrant visa petition by a qualified family member does not mean automatic issuance of the visa when the applicant appears before a U.S. Visa consul at the U.S. Embassy.
Visa petitions (in the Family and Employment-based categories) are filed with and decided by the U.S. Citizenship and Immigration Services (USCIS) – www.uscis.gov.
If the petition form is properly completed and supported by the required documents, and the petition is approved, the file is forward to the National Visa Center either for immediate visa processing (in the case of a petition filed by a U.S. Citizen to a spouse, minor child or parent, termed as “immediate relatives”) or for archiving (if the petition is for a family member in the Preference Categories (and therefore, subject to the annual quota and per-country limits –check our post on Visa Expressway - https://www.facebook.com/AmerikaAtbpTV/videos/vb.110143899069128/777670078983170/?type=2&theater
Upon completion of documentary requirements and proper payment (for petitions filed by U.S. citizens for immediate relatives) a notice of an immigrant visa interview schedule at the U.S. Embassy is sent to the principal applicant.
For other relatives (in the Preference Categories) processing of the immigrant visa can be started long before the priority date become current (See our post on State Department issues Two Priority Date Visa Bulletin Report - https://visacenter.org/index.php/page/487/state-department-revises-october-2015-visa-bulletin
During the interview, the applicant would be asked questions from the information contained in his or her file. If everything checks out, the visa is issued. If not, the consul must issue a written notice of temporary or permanent visa refusal.
The basis for refusing visas are contained in Section 212(a) of the Immigration and Nationality Act.
§ 212(a)(1) Health-Related Grounds - waiver exists for medical ineligibility..
§ 212(a)(2) Criminal and Related Grounds - waiver available for certain crimes depending on when committed and sentence imposed.
§ 212(a)(3) Security and Related Grounds - certain cases waived.
§ 212(a)(4) Public Charge - Joint sponsor for petitioner could be provided to overcome this inadmissibility.
§ 212(a)(5) Labor Certification and Qualifications for Certain Immigrant 212(a)(6) Illegal Entrants and Immigration Violators - principal applicants of specific petitions in the Family-based category may qualify.
§ 212(a)(7) Documentation Requirements (A) Immigrants (B) Nonimmigrants - Temporary refusal could be remedied by submitting the requested documents in the consular letter.
§ 212(a)(8) Ineligible for Citizenship
§ 212(a)(9) Aliens Previously Removed - family members in certain Family-based petitions could qualify.
§ 212(a)(10) Miscellaneous
You can check the individual posting for each refusal here in our website.