How to get a Proof of a Concord Well Kept

Now that you have consumed the soul of Vordt, you cannot get the hammer (until NG+).

To transform a boss soul into a weapon/spell/ring, you have to get the Transposing Kiln from the Curse-Rotted Greatwood (Undead Settlement boss) and give it to Ludleth of Courland (the guy who sits on the throne in Firelink). Once given, you can trade your boss souls with him.

Source : http://darksouls3.wiki.fextralife.com/Soul+Transposition

It's not just you, a lot of people have been having issues getting summoned as a Darkmooon or Blue Sentinels, and folks are pretty confident it's a bug, despite FROM saying otherwise. The good news is that there does appear to be a work around you can use, provided you're on PS4 or PC. You can find the links to the solutions here.

For PS4 folks have found that making a brand new account fixes the issue, although no one is sure why. Definitely worth a shot though. For Steam you need to make a new account as well, and then set up Family Sharing so that you can play Dark Souls 3 on the newly created account. You can transfer your save over to the new account) by replacing the save files in the new account with the ones from the old account, but some folks have reported that transferring the save didn't work, so you may be forced to start a new account if you want to do Darkmoon/Blue Sentinels coop.

So, in short, the whole thing seems to be broken and the only workarounds are kind of terrible, but there are workarounds. Once it is all working the range for being summoned is (source) (Host = Way of Blue player, Summon = Blue Sentienl/Darkmoon Player):

Host's Level - 10 - (Host's Level * 0.1) < Summon' Level <Host's Level + 10 + (Host's Level * 0.1)