I just finished the game and I didn't use my summon stones at all, so I was wondering if I could now go back and help people with bosses, or do I lose that ability once I clear an area? I tried messing around with it, but it seemed I couldn't put down my summon sign at all in some areas.

You can absolutely go back, but yeah there are areas no one can put a sign down like inside boss rooms. The problem you might run into is the game matches only in a certain range of your level/soul memory, I don't know if anyone has figured out how it calculates that range yet though.

As far as I can tell, there are a couple of bosses where you can't put down a summon sign. The two rat covenant bosses and the Giant Lord comes to mind.

I tried putting one down outside the door of the boss trio in the Lost Bastille, but it wouldn't let me. Maybe I was too close to the door.

Sometimes people will try and summon and it will fail, leaving you without the option to use it again. It happened to me yesterday. I think I just backed out of the game, and when I went back in I was able to use it again.

You can put your sign down in most areas. Warp to some place and place your sign a few feet from the bonfire to confirm that you can place it down. If you are in the covenant Company of Champion you will not be able to use your white sign soapstone. You can leave the covenant via the cat and be able to place your summon sign down normally.

Putting your sign at the nearest bonfire is a good option too. But if you finished the game and went back to a boss so early in the game (Like the 3 Halberd-wielding bosses in Lost Bastille) you might have any luck. I'd try sticking with the last 2 or 3 bosses you defeated at the end of the game.

Also keep in mind that summoning is based on Soul Memory. If you beat the game, people in the Lost Bastille might not be able to summon you.

@strainedeyes said:

And you can only play with people within the same playthrough setting -- not NG+ for example if your save is still left on your first playthrough. Lost Bastille will be impossible to do in coop for you now, as will most areas in fact given you've beaten the game. Though there's always people looking for help against the Throne Defender & Watcher and of course Nashandra, which will probably be the only bosses left that are applicable for you at your level.

Nonetheless, if you're after playing some coop for a variety of bosses, I think it's best that you start NG+ and put down dem signs!

Darklurker from the Pilgrims of Dark covenant must also be beaten solo.