Out of curiosity, are your encounters all built in the rigid "here's an enemy, now kill it" approach? I've found that the much more interesting encounters stem from conflicting goals between the players and the "enemy", and then this also allows you to throw more dangerous monsters at the players.

For example: an encounter with a gorgon which would be beyond deadly for the party in a straight-up fight to the death. But if you instead task the party to get past the gorgon (over a bridge or through a doorway), then it becomes a lot easier. They can make it difficult by trying to kill it, but they will have a goal to achieve in the encounter. The party's goal is to get past, the gorgon's goal is to stop anyone getting past. direct conflict, but not necessarily to the death.

It sounds to me like you need to think through the terrain and tactics for the opponents. a straight fight with 10 bandits might be a medium encounter, but as soon as you have them ambush the party in a ravine, then it becomes complicated - if the whole party relies on running up and hitting things, then when 10 guys with bows appear on the top of a cliff, block your exits and rain arrows down on you, it becomes a near impossible fight. If the whole party is ranged, then the ravine element is irrelevant. 

Also, giving consequences which are beyond "you won" or "you died" makes the combats more meaningful, even if they are easily dealt with. The party might be on a wagon, and the wagon is hit by flaming arrows, causing the horses to bolt, all because they didn't pay the bandits a toll to pass the ravine. they might then find that a giant has captured their horses, and they can choose between a really hard encounter with a giant to get them back, or proceeding on foot.

You also need to consider the spells the players have and their effects. They can levitate one gorgon, so you know giving them 1 difficult monster without any buddies won't work. More numerous enemies, with better tactics, could be all you need to make things a bit more difficult for your players. they rely on magic? Find antimagic monsters. Have snipers in the woods with bows & arrows which target the concentrating spellcaster.

know your players, ask for a copy of their character sheets if needed. learn their characters abilities and use monsters that they don't have an exact counter to, but also doesn't completely counter them

The trick is to listen and observe when you're running these encounters. If you players breeze through it...why? Something could be super deadly with undead but your cleric get lucky with turn undead and you horde of the dying mob is out the window. 

You hard/scrape by encounter...why? Like a blue dragon for a party that is mostly grounded and is gonna be tough as those blue dragons love to stay in the air. 

So what I tend to do is I see what my players can and can't do and start from there. Attack their strengths and sell it so they believe it. Also as for feedback. Some encounters I run I feel that it was too easy and my players say their hearts where in their throats and visa versa. Never under estimate what a little feedback can do. Also, make the environment another part. They should fear the map as much as the monster.

My motto is that I tend to cater the fights for them. If they like it hard I throw everything at them. If they are more chilled I throw narrative in the combat.

This is what I do. I'm not tactically savvy so I tend to up my monsters in power. Be aware of your strengths and shortcomings and work it into your encounters.