When an adjudicator determines that a final approval determination cannot be made immediately, they may invoke Section 221(g) of the Immigration and Nationality Act (8 U.S.C. 1201). Section 221 is titled "Issuance of visas." and the (g) designation speaks directly to the "Nonissuance of visas or other documents".
"(g) Nonissuance of visas or other documents
No visa or other documentation shall be issued to an alien if (1) it appears to the consular officer, from statements in the application, or in the papers submitted therewith, that such alien is ineligible to receive a visa or such other documentation under section 1182 of this title, or any other provision of law, (2) the application fails to comply with the provisions of this chapter, or the regulations issued thereunder, or (3) the consular officer knows or has reason to believe that such alien is ineligible to receive a visa or such other documentation under section 1182 of this title, or any other provision of law: Provided, That a visa or other documentation may be issued to an alien who is within the purview of section 1182(a)(4) of this title, if such alien is otherwise entitled to receive a visa or other documentation, upon receipt of notice by the consular officer from the Attorney General of the giving of a bond or undertaking providing indemnity as in the case of aliens admitted under section 1183 of this title: Provided further, That a visa may be issued to an alien defined in section 1101(a)(15)(B) or (F) of this title, if such alien is otherwise entitled to receive a visa, upon receipt of a notice by the consular officer from the Attorney General of the giving of a bond with sufficient surety in such sum and containing such conditions as the consular officer shall prescribe, to insure that at the expiration of the time for which such alien has been admitted by the Attorney General, as provided in section 1184(a) of this title, or upon failure to maintain the status under which he was admitted, or to maintain any status subsequently acquired under section 1258 of this title, such alien will depart from the United States."
Typically, when a 221(g) is received, the case goes into administrative processing, which is when an agency or department is awaiting additional information. Another possibility is when the consular officer simply refuses the visa and sends the case back to USCIS for reconsideration. When a case is in AP, it could take 60 days or more for a determination to be made.