Can you say what sites it can't capture from? Are you able to post any example links?
The next time this occurs, can you attach the log file? You can find it by going to the Preferences and clicking the Find Log File button.
Friday I was trying to capture the live stream from Outside Lands festival. Today I was trying to record from Amazon music which uses Twitch for concerts. It was actually a taped show and not a live show, but it wouldn't capture the video.
Unfortunately we can't capture twitch video at the moment (at least on the Mac). This is a known issue and we will add support for it in the future if we can. Sorry I can't be of more help regarding this particular site. Your best bet in the meantime would be to capture it using a screen capture app such as Jaksta Screen Recorder.
Let us know if you continue to have issues with other sites. If possible, posting a link to the video as well as your log helps us troubleshoot it.
I have been unable to get the app to record streams being broadcast live. Rips from YouTube and other sites without issue. No luck on shows being broadcast live.