Sometimes it heavily depends on RNG whether a run is successful or not. What has been a piece of cake the first time might not work this way the next 10 times. 

How many times did you run this with your team? As you are not losing keys when trying to run DT...it might be worth trying several times.

Affinity probably changed.

I think was still Force afinity ; but actually was not 2 weeks ago but 2 rounds :)

Last round was the one with new boses - dark fae and bomb dude 

I will try other formation for this boss

Affinities make a huge difference, but if your 2-3 weeks is correct, it looks like you're actually comparing Dark Fae and the Scarab King against each other. They are very different bosses with very different mechanics. They are not of exact equal difficulty and each requires a very different team setup to be successful.

In Dark Fae you need a "Go First" arena style team. In Borgoth you need shields and regeneration. Frost Spider you need reliable HP burn/Block Revive. And so on for the other bosses.

I was comparing same boss on the same level : Borgoth on force on 100 lv on normal DT .

I saved the team I was using to beat him 2 rounds ago ( without any problem I might say).

Today - same boss, same afinity and same team ( without changing anything on champs) - maximum i was getting was to let boss with 30% HP .

Tomorow I shall check different setup to kill the dude :)

Didnt notice a difference to be honest? full autoed through normal, slowly working my way through hard.

Sometimes bad rng can mess up a run.

Did you drop down in the arena?  Arena tier effects global stats.

Nope - same G4 as I was 2 rounds ago.

if no1 saw any difference maybe was just lucky round for me last time

i will continue today to see how i can finish the dude :)