Keep in mind that sometimes even if the DM perceives the encounters as easy, the player have instead a blast and no particular adjustment is needed.

Said so, a few observations/suggetions.
- If players rolled for stats or you gave them items/free feats they are obviosuly strong and you must keep that in mind.
- If players are smart they will use their abilities in a synergic way, making the encounters very manageable. 5e doesn't assume players are masterminds and some tactics are really effective
- Obviously be sure they are playing by the rules and not exploiting a rule that your party didn't get (for example, how surprise works in 5e, with no round of surprise)
- CR is a bit... Swingy, but I can assure you that is a valuable tool to avoid TPK... In most cases. But if you choose your baddies and move them IN A SYNERGIC WAY, they will start becoming MUCH MUCH more strong! For example, don't always go for mindless attacks, use the help action, go call for reinforcements and retreat if a battle is lost.
- Use terrain and the environment as a strategical advantage. Often PCs will be in the enemy territory, and there your baddies should have the upper hand, preparing traps or placing themselves carefully.
- Always remember that the point is not countering the PCs. If they builded themselves to be strong AND THE TABLE IS HAVING FUN (yourself included) no adjustement is needed. You need to do something is a tactic is becoming boring or too much repetitive, then use smarter enemies.
- There is no need to drain completely the PCs using fights. Exploration has its own challenges. A wizard may need Levitate to go past a chasm for example, or a wizard might need to Shatter a cave entrance.
- Forget the 6/7 encounters rule. Just RP awesome fights with smart enemies and try to prepare memorable bosses. That in the end will be what they'll remember.


Hope this helps.