Thanks for the tips.
We actually got my sister to joint sponsor. I have the copy of my own part but did not copy her part. In any case, I suspect that partially it is because I lived abroad and did not have US income. I did file a tax return in the US so listed the amount as "income" on the form, but I ultimately did not pay taxes on them in the US since I paid abroad already. I supplemented with assets though. My sister however does have US income so not sure why that doesn't work as well...
Will double check but maybe also seek legal advice on this...