Contact
PhraseLoop is built and maintained by one person, so messages come straight to me. Bug reports and feature requests are genuinely welcome — most of what the tool does now came from someone pointing out that something was awkward.
For a bug, it helps enormously to include your browser, whether you were using a YouTube link or your own file, and what you expected to happen. If a specific video failed, the link to it is the single most useful thing you can send.
Elsewhere
A note on videos that will not play
The most common report is a YouTube video that plays on YouTube but not here. That is usually because the owner disabled embedding, the video is age-restricted or region-locked, or a school or workplace network is filtering YouTube. Trying the same link on a different network is a quick way to tell which it is.