Just wanted to share this in case anyone else has this issue. One of my sites is hosted on SiteGround and I registered it in the Adsense web console. I had the ads.txt file there for months but Adsense was still saying that it couldn't find it. I was able to easily bring it up in the browser. It was there for sure. I finally discovered the problem and fixed it. Since I did not have a robots.txt file on my website SiteGround was using some mechanism to default one for me. However, their robots file had a crawl-delay of 10 seconds set for all user agents. Turns out this is what was killing the Adsense crawler. Added a robots file and set the delay to 1 second and its all good now.