Personally I think the difficulty settings for encounters should just mean what they say on the tin.
I expect an easy encounter to basically be a cakewalk, people can just kind of clown around and make it through.
I expect hard to be....hard, aka if my players aren't bringing it they should expect someone to die.
I expect deadly to be...you know....DEADLY, I am assuming that someone is dying in this fight, and only expert play will prevent a TPK.
What I don't expect is for my players to walk through 3x deadly encounters with basically not a scratch on them (which they do commonly).
There are far too many variables to have a system that will work for everyone. Some games have people that aren't tactically adept, some use point buy, some roll until you have at least 1 18 before racial adjustments. Some have magic items up the wazoo, some don't have magic items at all. Some have 1 encounter per long rest, others have 10 while designing combats with environments that favor the enemy. I can fairly confidently say that a 3x deadly encounter would end up with a TPK with my group on a pretty regular basis if I wanted it to. Heck, high end hard encounters could probably kill off a PC on a regular basis if I really worked at it.
I can't tell you why you have a hard time challenging your group, I can only tell you that I've had no problem challenging multiple groups. With only rare deadly encounters.
P.S. Deadly just means a PC is likely to die. Not guaranteed, not a TPK, just likely.