In case anyone runs into the problem I had:
I was having the same issue many of you ha{d|ve} - forms were showing up on app.netlify, but submissions were blank. After trying all the fixes in the various places on the internet it still wasn’t working.
I finally realized my own submissions were being filtered by Netlify’s spam-filtering service provider. After reverse engineering the spam filter tests through trial and error I found that if you have a form field named “twitter”, the spam filter Gods REALLY want (need) your entered data to start with an “@” character. Wow! Lost more than a day of my life to these spam filter Gods.
tl;dr - Make sure you enter good data while testing. For me that meant including “@” in my “twitter” field.