Based on your response, it doesn't seem like the hold up is based on visa availability?
She only has Korean citizenship.
NVC's response seemed boilerplate so I'm not sure if they'll provide specific details on her case and explain the delay but maybe I should try asking. I've pasted NVC's response below for reference.
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Greetings,
This case is documentarily complete, which means the NVC has all the documents we requested. Your immigrant visa case is waiting for an interview appointment, where a consular officer will adjudicate it.
The U.S. Embassy or Consulate provides the National Visa Center (NVC) with a list of available appointment dates and times. We will schedule your case for an interview appointment when an appointment date is available. We schedule appointments on a first in, first out basis. After we schedule your appointment, we will forward your case to the U.S. Embassy or Consulate General in SEOUL.
We cannot predict how long it will take to schedule your appointment, but we will notify the applicant, the petitioner, and your attorney (if applicable) when an appointment is scheduled. Your case will stay at NVC until that time.
You can check current scheduling dates at: https://travel.state.gov/iv-scheduling-status-tool
For the most up-to-date information about the operating status of the U.S. Embassy or Consulate, please visit their website at https://usembassy.gov.
Information about preparing for interview appointments is available at these websites:
https://nvc.state.gov/prep
https://nvc.state.gov/prep/espanol
Important: Do not make travel arrangements, sell property, or give up employment.
If you want to request that we expedite this case, please provide us with more information. Please send your request with the supporting documentation (such as medical records, a letter from a medical doctor, or deployment orders) to NVCexpedite@state.gov.
Enter the case number in the subject line of the email. Within the email itself, please include all of the following:
· The reason for the request
· The petitioner's name and date of birth
· The beneficiary's name and date of birth
· The beneficiary's two local contact phone numbers.
When we receive this information, the Department of State will review your request to expedite your case.