Post screenshots of the champions who you think meet the criteria, so we can see what you're doing wrong (and yes, in 99 percent of cases it's the player overlooking or misunderstanding something, not a bug...). 

Yes, individual screenshots of the champs would definitely help. Maybe you can also provide a screeshot of the specific challenge?

i

for example the above screenshot is a a champion named preacher and on the right hand side you can see lvl 3 sets. each of the artifacts are at level 8. please check and let me know if this is not correct. this is how it worked once but now no matter how many more I add, the status still shows 1 out of 4 complete

Can you add a screenshot of the challenge text itself, please? The wording may give us an idea what you may not have completed, apart from the need to equip 4 champions in this way, not just a single one.

the second champion is kael in the screenshot below

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You need 2 of each of those sets to complete a set. That one has only one set completed, and you need them on 4 champions at the same time. To complete the quest.

the screenshot of the challenge is attached 

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If I understand , I need the artifacts to belong to the same set, in my case all 6 belong to different groups, thanks I will try to pick 2 from the same set and see if that helps

The second pic got one complete sets i believe. Shield, Gloves, Boots and chest doesnt match, and boots and chest are from 4-piece sets. Easiest to complete that quest is to use the life, def and, attack sets Like in the first pic.

You can mix sets as long you got two of each on the champion And be sure to pick 2-piece sets. Try it out and post again if quest doesnt complete for you. A tip is also that right above your hero power in the pics you posted you get an icon for each complete set you got equipped. You got a shield icon for defense set on first pic and a green icon for resilience set on second pic.

I tried with a third champion and used artifacts from the same set, it did not work, see screenshot

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Should I use 2 artifacts from one set and 2 from the other and 2 from a third set, let me try that. althought I think using all 6 artifacts from one set should also work, or is that incorrect?

Lifesteal is a 4-piece set. As you can see on the icon you got one set bonus. use life, def and attack sets they are 2 piece. So using 6 pieces from any of those work, but you cant use any 4-piece sets.

Use any of these that has a (2).

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That worked, thanks guys, I was able to figure it out. back to the fabulous game of raid.