YouTube Live Chat can take up to 24 hours to appear on your YouTube Live videos. As long as your video is public when you stream it and you have Live Chat enabled on your channel, live comments will show up after YouTube has processed the video.
It could also be that for some reason the Live Chat Replay is turned off for your channel. Here's how to turn it back on for future broadcasts:
- Head over to your YouTube Live Dashboard.
- Press the "Edit" button at the top right.
- Click to the "Customization" tab, check "Live chat replay", then press "SAVE"
Now your broadcasts going forward should have the live chat replay available when re-watching your livestream!