A Bloodborne bug has prevented players calling on help to take on one of the nasty early bosses.
UPDATE: Turns out you need to open a specific gate in order to summon on this area - more details.
Bloodborne bosses are significantly easier to take down in co-op.
Unfortunately, at least one of them is proving difficult to find help for - and not just because not many players have obtained the Small Resonant Bell yet.
Multiple players have reported issues using both the Beckoning Bell (summon a co-op partner) or the Small Resonant Bell (volunteer for co-op) in the Cathedral Ward, the district where the Vicar Amelia boss battle takes place.
Both bell items are greyed out, as if the player were offline, and attempting to use them produces the error animation.
Normally, players cannot summon in areas where they have already cleared the boss, or volunteer from inside a boss arena. But apart from that, there doesn't seem to be any reason why this particular boss should be blocked from co-op - especially as many other players have successfully hosted and joined co-op sessions there, and affected players can summon and be summoned as normal in other districts.
It's possible this is some odd quirk of Bloodborne's, as From Software delights in not explaining the rules of its games very clearly - perhaps you can't summon for a boss if you've summoned for two or more in a row, or something like that (although it seems unlikely, since you can't volunteer, either). Or perhaps you need a higher total Insight to summon in more advanced districts.
On the other hand, it might be a bug - and if so, let's hope it's fixed in the upcoming patch.
We've emailed Sony to see if we can find out what the deal is.
If you need some help smacking the Vicar Amelia, check out our Bloodborne guide and walkthrough.