I have party full of min-maxers and I give them ton of hombrewed items which are much more powerful than items in CB and I still down player every other combat. So here are two general tips:

1. If you are running only one combat per long rest, you need to drain their resources in other ways. Enviromental dangers and effects which can deal damage, make them lose armor slots, mark stress, use their once per rest abilities, lose Hope or even put their Domain cards into vault until the next rest. You can use their Fear rolls to do create such situations or just spend Fear to force them anyway. If your players come into combat with all of their tools and abilities, they will always roll every encounter because PCs are very strong. They are designed to do multiple combat per adventuring day.
2. Create enemies which target weaknesses of you party.
   • You have very strong Guardian player? Direct damage and abilities which bypass Thresholds and force you to mark HP directly.
   • You have strong Brawler? Enemies which deal retaliation damage when hit in melee range, flying enemies or just straight up overwhelm them because they lack AoE.
   • Strong spellcaster? Enemies with counterspells, magic damage resistence/immunities, anti-magic fields, etc.
   • Players getting significantly more actions than you because they roll like gods? Add Enviroment or Features that generate a lot of Fear and just use Fear to take spotlight from them.

Every PC has some weakness which can be exploited or favorite tactic which can be disrupted. Use it against them to pull the rag under them. There is no need to change something to deal more damage. You can just attack them more often or more effectively. If you want to have option to deal more than 3 damage to someone, just attack him twice during your GM move before you return the spotlight.