Some of my players have said the campaigns I give them are too easy. Can anyone help me design one that will be harder? (They are level tens)

Let’s say there are 4 of them. Using the 2024 encounter rules, 3100 xp for each in an hard encounter, giving a total budget of 12,400 xp.

One Adult Black Dragon

One Merrow

and 4 Sahuagin Warriors

Use them all in the swamps with plenty of terrain and water, hit them hard and without mercy. But remember, the encounter will be far easier if it’s the first fight of the day, pepper in smaller fights to use up some resources.

Just up the difficulty of the encounters by a step or two, and DO NOT throw in random dragons like @CharismaCaster said.

You are the DM.  You do not have to stick to what the book says about monsters.  Adapt them, change them.  Know your group you are going against, understand how they battle, then target those tactics to change them up.  

It was hard, alot of work but one of my best encounters I gave was against another party.  I created characters, and I used them like I would if you were playing.  I also added in Villian Actions, (Matt Colville).  

But adjust raise hp, raise AC, give them special abilities, in middle of battle, reinforcements come in.  It is your world.  Have fun.  

Create more complicated puzzles and riddles for them. If it is the combat that they find too easy then add a couple more monsters and place them strategically around the area. It is your world your rules, Your call .

Environment is a great way to increase difficulty. 

So, the player characters are coming along a long hallway. At the end of the hallway is a door which opens toward the party. Inside are a load of magic users who are guarding the door. Once it opens, they send a lot of magic attacks at the party. The only way for the party to get concealment is to close the door. There's nowhere to hide. Same principle if the guards are ranged fighters. 

The party are in a dungeon where the ceilings are only 6 foot high...this means that martial characters cannot use their two-handed weapons. Even standard weapons might be at a disadvantage (see Wave Echo Cave in Lost Mine of Phandelver). 

The party are traversing a mountain path, a large flying monster swoops in and out, but the party have nowhere to hide so are entirely exposed.

The encounter takes place in a dense forest, pushing through the brush makes it difficult terrain, but the enemies are able to climb and have ranged attacks. The enemies are up in the canopy.

Think about where encounters take place. Never give a party a big wide open space unless you've got good reason to. 

Another tack to take is to entirely ignore 2024's scrapping of the adventuring day. A fight in D&D will generally be minutes of in game time, if not mere seconds. So, stuff those encounters in. Wick away their resources. The adventuring day made sense because it made difficulty a matter of attrition. The party have limited resources. So as a day goes by and more encounters occur, they have fewer and fewer resources. With 2014's adventuring day, the likelihood was that towards the end the party would have few spell slots, fewer hit dice, and in general be less powerful. It was a built in difficulty factor that DMs could use. I know that not every DM agrees, but the adventuring day genuinely was a good mechanic.

In general, getting the party to burn through those spell slots, burn through those hit dice, superiority dice, and suchlike, made for a greater difficulty level as the day progressed. So, you can make something more difficult by simply packing in more encounters between short and long rests. And don't allow too many short rests. Enforce too that a long rest can only happen once per day. 

Monster effects and plans can be great too. The cave that the party are in has been rigged with traps by the local troupe of Kobolds who live in the area. If the party enter combat with the Kobolds, they know where all the traps are and lure the party into said traps.

Crossing a canyon the party enter into combat with the local thugs and highwaymen. The highwaymen, have the advantage of knowing the terrain. They've rigged the bridge across the canyon and once the party are halfway across the highwaymen force the bridge to collapse.

The wildfire druids of the nearby area have set light to the forests. In trying to stop them the party need to contend with the nearby animal and plantlife who actually side with the druids plan to force regrowth and renewal. All of a sudden even getting the druids means finding a way through an environment thick with thorns, poisons, aggressive beasts, and thick pools of mud. All the while, the druids could ambush at any time.

Alright, thank you all for helping!

If the players are saying that your combats are too easy, ask them specifically what they find displeasing about them and try to adapt it to what they think would make it better. Communication between the players and the Dm is a very important part of any roleplaying game.