Parents visitor visa dropbox, got 221g and called for interview

[qquery]

Hello,

My parents Visa was about to expire soon, so i applied for renewal via dropbox as they were eligible.

After the dropbox, my mom passport came back with 221g(white slip) asking her to come in for an interview with the consular officer on nay day from 10-11. And her visa had a stamp " cancelled without prejudice". But my dads passport is still not returned. the status should its still with us embassy/consulate,

worried why is the 221g form? she visited around 5 times in the past 10 years and mostly was 6 months long and frequently( within 6 months ) once.

Also the last time when she went to interview 10 years back, both were working but now both are retired adn they are self sponsers. Could that be an issue?

what do you think could be the reason for 221g and what kind of questions can i expect in the interview.
Any documents that she needs to carry? ( slip just said bing the slip and passport to the interview and no appoint required)

Any help appreciated. Thanks

[qquery]

Actually she retired recently and after that she was with us 6 months. Not while she is working.

 

And also its a blue slip. not white. does it make a diff . not sure

Edited October 4, 2019 by qquery

[qquery]

Thank you for your response.  Sorry i missed you msg. Yes she did not visit for 6 months while she was working. only after she retired, she did. 

And everytime she was here for 6 months. we had some occasion like family wedding or something going on and not for any under the table work as you mentioned.

Edited October 9, 2019 by qquery