I have been a DM for about a year and am trying to get better at it.
I have been using a houserule in which adjacency provides a bonus to AC. I use it for monsters too. I have 3 players. All of them have their main stat at +4. One of them even has a +3 modifier on 3 other stats. They are a Rogue (the with so many +3) Barbarian Half-Orc and Cleric. Their AC is 19 or 20.
They are at lvl 6 and to be honest, I don't think i can make anything challenging anymore. The Rogue takes only half damage if something is able to hit him (Orcs were good at it with their +4). So does barbarian. Their dmg is about 15-30 per round. But almost nothing is able to hit them or is too powerful to be able to kill them. I tried to use many not so strong enemies but it was only a matter of time for them.
For about 3 months, I have been using more and more things for the purpose of increasing challenge, because I am not enjoying the game, since they automatically win.
Right now I am trying to use more dangerous monsters, but they suddenly (except for the cleric) feel like my intention is to kill them, and that I am just trying whatever it takes to do it. I am raising difficulty, because it's not fun for me, if the outcome is clear.
In my opinion, adventuring is supposed to be lethal. That's why peasants don't do it for a living. I think that no matter what they power, good adventure should have a chance to kill them so that in order to achieve the goals of the adventure, they need something other than good stats.
Is my mindset too much "I am trying to kill the players for my pleasure" or in line with "I am trying to make a challenge that ALL of us can enjoy."? How can I increase the difficulty appropriately?