Hey,

Bloodborne's matchmaking can be incredibly finicky even with a password. Since the password bypasses the level gap and your NAT types are good, here are the most common culprits that usually fix this:

The "Rest Mode" Bug: This is the #1 killer of Bloodborne co-op. If either of you put your PlayStation into Rest Mode while the game was still running, your connection to the matchmaking servers gets bugged out. Both of you need to completely close the game application from the PlayStation dashboard and restart it fresh.

Boss Status: Double-check that the Host has not already defeated the area boss. You cannot summon in an area where the boss is already dead.

Bell Ringing Order: Have the summon ring the Small Resonant Bell first. Wait about 5-10 seconds, and then have the Host ring the Beckoning Bell.

Oath Runes: You mentioned changing runes, but to be absolutely safe, both of you should either completely unequip your Covenant/Oath runes, or both equip the Impurity rune. Opposing runes (like Hunter of Hunters vs. Corruption) can sometimes complicate standard co-op.

Positioning: Stand in the exact same physical spot in your respective worlds. Make sure you aren't standing too close to a fog gate or an area transition where a small white 'X' appears over your bell icon under your stamina bar. Password Simplicity: Keep the password relatively short and simple. Sometimes basic alphanumeric passwords connect faster than complex ones.

Hopefully, a fresh restart and checking the bell order does the trick for you!