H4 Visa Dropbox - issued 221g

[raj21d6]

Hi,

Here is my situation:

Last year while H1b extension was in processing, my spouse on h4 was in India. While she was in India, h4 extension cannot be filed.
She travelled back to US (with existing valid h4 visa) after H1b extension approved.

And in USA, her i-94 extended to the date of H1b visa

Later H1B came to India for visa stamping but she remained in USA, her visa expired but with i_94 valid.
This year she went to India for H4 visa dropbox and got 221g white slip, asking to appear for interview.

Can anyone please let me know what might be the reasons and what to do.

Thanks in Advance
Raj