In Byrgenwerth, I could summon the Younger Madaras Twin and Old Hunter Henryk to help defeat Rom, The Vacuous spider. However, both got stuck in the door to Master Willem's balcony thus they were unable to help. After Rom's spider bros exterminated me, I tried again with just Old Hunter Henryk. He got stuck as well, unable to pass the mist.
Note that I ended up defeating Rom, The Vacuous Spider easily enough without them. Others might encounter the same bug…
The exact same thing happened to me when facing Ebrietas Daughter Of The Cosmos and Mensis Scholar Damian.
How do you get those hunters to help you in the battle?
Do be aware that the boss's health does still scale even if the Old Hunter is not with you! This makes this a horrid bug.
Best Answer
With Ebrietas Daughter Of The Cosmos and Mensis Scholar Damian. I tried to reload the game. This did not work. I fought Ebrietas and got mashed into smithereens. I tried the summon again, as I have around 70 insight and nothing to spend it on, and this time Mensis Scholar Damian did cross the threshold.
The "fix" appears to be: Die and let the game reset.
It might be worth taking a backup of your save file, trying the summon, then restoring the backup if the summonee get stuck. PITA but probably will never get patched. This will avoid you losing Insight pointlessly.
Or just git gud and kill the boss solo…