I’ve noticed this issue as well recently – even when parsing known “good” sites.
Coincidentally, I was also having issues with services like IndieWeb.xyz parsing my own site, even though every parser and validator I threw at my site picked up the microformats without issue – including my local install of Parse This. In the end I tracked the issue to the WordFence for WordPress security plugin tagging the incoming request as a hostile bot and rate-limiting/blocking it in response. Infuriatingly, this was working fine for the previous few months. I’ve turned off WordFence and my site seems to get picked up properly now.