Build (sometimes!) not triggered for a pull request

We have been using integration with CircleCI, side by side with Appveyor and TravisCI, for our project here:

The problem we experience that sometime (not always) CircleCI is not triggered for new pull requests. We did not make any changes to CircleCI setup, and for the latest PR it does not work:, while it worked just fine for the previous PR, and we did not touch CircleCI configuration in between:

Can someone help us understand what is going on, how to fix this, or what could be a workaround?

It appears that in this particular instance, build on a PR was not triggered had something to do with the privileges of the user who did the PR. Once I made a PR for the same fork using my account, CircleCI build was triggered.

Hmm, as I think more about this, PRs from the same user did trigger builds on CircleCI in the past … Very strange, maybe something changed on CircleCI side in terms of handling PR submitters permissions?