Advice for new DM with struggling players(balancing issues)

I´m curious: Has it been like that from the beginning or has it switched around at some level ?

I´m pretty new as a DM myself and I´m currently struggling with quite the opposite. I had my group of 4 (fighter, paladin, rogue, sorcerer) fight CR3 enemies at lvl 3 (for example an Owlbear and a Grell) and they barely were any challenge. Most of them died in the second round of combat and they just landed one or two hits on the players.

We play 5.5, so that might be a factor, but we don´t use the weapon masteries. In addition, I always use the maximum HP for all of the monsters instead of their average as shown in the monster manual. And the players barely have any magic weapons.

Last session, the players leveled up to 4, and fought a CR5 monster (a revenant) with three ice mephits (CR1/2) and while it at least took longer than 2 rounds, it still wasn´t really a challenge.

Quote from Eugel >>

I´m curious: Has it been like that from the beginning or has it switched around at some level ?

I´m pretty new as a DM myself and I´m currently struggling with quite the opposite. I had my group of 4 (fighter, paladin, rogue, sorcerer) fight CR3 enemies at lvl 3 (for example an Owlbear and a Grell) and they barely were any challenge. Most of them died in the second round of combat and they just landed one or two hits on the players.

We play 5.5, so that might be a factor, but we don´t use the weapon masteries. In addition, I always use the maximum HP for all of the monsters instead of their average as shown in the monster manual. And the players barely have any magic weapons.

Last session, the players leveled up to 4, and fought a CR5 monster (a revenant) with three ice mephits (CR1/2) and while it at least took longer than 2 rounds, it still wasn´t really a challenge.

 Are you following the Adventuring Day principle? One encounter per long rest will never be a challenge. 6 in a row will.

"Them running away causes another issue: them sometimes thinking/trying to do 1 encounter -> tiny hut/escape the dungeon -> long rest -> go back in. like in a video game."

I feel like this may point to the root cause of all the tpk's.

For a dungeon crawl, runnijg away and trying again later is EXACTLY what the party should do.

Thats not an "issue". Thats the party being smart about their limitations. If they happen upon a room with 4 bugbears, combat starts, they kill 2 and then run away because a player goes down, then there should be 2 bugbears left if they come back. If its a guard post at an important location, a dead guard may be replaced. But the monsters in an encounter generally are finite and fixed in number.

If you keep resetting the rooms if they leave as if the npcs are infinite, and if you think that them leaving and coming back is itself an "issue", then you are giving the party only one option: throw themselves at a room until one side dies.

Now, it may be true that sometimes the situation does not allow the party to delay. The cultists are starting their ceremony and the world will end in 60 minutes. But that should be done with care becaue it removes certain options from the party. If it happens every time, it will feel like railroading

In a dungeon, the general idea is that there is a finitr number of enemies in the rooms that comprise the ecosystem of the dungeon. The party should be allowed to try to choosr what path through thr dungeon they take, when to fight, when to run. But if they kill 2 bugbears out of 4 then run away to rest or heal, then there should only be 2 bugbears left in that room when they come back.

Now, combat will cause monsters to move around, come to aid other monsters and so on, but that should be limited by how much noise is made and how far aesy the combat is from the monsters. If they cant hear it, they should stay where they are.