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Two potential causes

1. If you miss the first alert for a thread, you won't get another one. This is to prevent you getting spammed with alerts
2. If someone quickly replies a second time and it gets merged and includes a mention of you, that won't trigger a notice. We've reduced the frequency this would happen. But it's still possible.

You can visit here to see what you haven't read: https://forums.macrumors.com/watched/threads

arn
 
Which setting please?

Another thing - I had a lot of watched threads so I recently deleted them all and started to add back more. That seems to be when these issues started. Now I have six watched threads but at the top of each page there's only a "1" next to Watched.
 
The number should represent how many threads you have that are unread. Does it not match up?
 
I’d just wait and see. Seems to be working. Maybe it was some temporary issue.
 
The number should represent how many threads you have that are unread. Does it not match up?

So just to follow up I came to the forum now and it showed 2 unread watched threads. The last posts in those two threads were about 2.5 hours ago yet there were no alerts shown at the top right of the screen. Is there usually a delay in showing the alerts or are they fairly instantaneous?
 
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