What do I do with a party that is much stronger than their level?

We’re playing D&D 5E. The party consists of 5 players, all currently level 11 (however, I’ve noticed this problem since they were level 8).

I’ve been using the Goblinist random encounter generator and the Kobold Fight Club encounter builder to build encounters. However, the party has been slicing through everything I throw at them.

For example, when they were level 9 goblinist suggested putting them up against 6 shambling mounds and that it would be a deadly encounter. One character took a little damage and a couple of the magic users used some spells (fireball was the most important one). It was clearly not challenging at all.

The most recent encounter (they’re level 11 now), Kobold Fight Club suggested two guardian nagas would be deadly. Since I’ve observed the group is overpowered, I enhanced the nagas with legendary actions (one had movement; the other could cast any spell). Two rounds later, the nagas were dead and the party again barely scratched.

I don’t think I’ve been overly generous with equipment. They do all have +1 weapons, but that seems reasonable since they’re level 11. The two fighters still have their starting armor, although the dwarf has a brooch of shielding. The elf has some magic silver arrows, but most of the time he just uses his regular ones. The Wizard has fireball, which I’ve nerfed by putting them near Varenrood Loch. The Loch is magical and any fire magic within 200 miles of it has a 40% chance of fizzling out. He now polymorphs himself or animates objects and seems to still do more damage than anyone else. The cleric isn’t even needed most of the time and just sits back and watches the fight.

Normally real life tears a group apart before we make it above level 6, so maybe it is just me not knowing how build an encounter for this level. Am I using the wrong tools to build the encounter? Is there anything I can do to get their power to match their level?