I've never gotten alerts to all threads I'm "watching".
From what I've seen, the logarithm seems to stop notifying me about threads for which I haven't commented in or liked posts in recently.
In other words, I could be wrong but it seems to me that if I just regularly read updates in a thread, but haven't liked a post or commented in the thread in a certain amount of time, then I stop getting notifications for that thread. Regardless of if I'm still "watching" it or not.