If you cleared the boss of an area you cannot use beckoning bell. If you cannot access the boss, you cannot use the bell. So various areas it's not able to be used and it mostly seems tied to if the boss is dead, if you can do the boss or if you are in the right area.
Also for the new page, posting this again so people see it.
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Seen a lot of rumors of this and tested to test myself.
With 14 and 27 insight there was a noticeable difference in the two monsters i tested in Cathedral Ward.
Video below shows the difference.
https://www.youtube.com/watch?v=-cKEM7NuyQw
Lantern guy gained eyeballs on his lantern and he now casts a ranged Spell.
Sycthe guy gains an enchantment on his Scythe that burns you.