Anyone having times where they can ring the bell and times when they can't? When I use it, it seeks out an available player but often fails to bring anyone to me. I've got it to work once it four times playing. Also, the bell goes inactive after using it as if it can only be used once for a certain time.

Once you activate it, it continues to search for connections as long as you remain in the same area and don't die. There's no need to use it again because it isn't an instantaneous thing, it's continuously searching. You'll see the bell icon in the top left hud that indicates it is searching and you'll also see a pulsating aura around your character's feet. It will continue searching until you die, leave the area, or enter the boss room.

If the bell icon in the top left corner is crossed out, or if you are unable to activate it at all, that means you are someplace summoning isn't possible (usually boss rooms, safe zones at the beginning of an area, or places that for one reason or another just don't support summoning). If you see the bell icon get crossed out, go back to area where it wasn't to continue looking for co-op partner.