Unfortunately, it's so easy to get caught up in making your MC great, and forgetting that they're not the only person on the planet. I'm sure we've all seen examples of stories where the main characters are great, but everyone else seems to fall flat.
We can't all be superheroes, but everyone can be a hero.
What do I mean by that? You can't make everyone a main character. The story can't be about everyone. But that doesn't mean you have to treat all the supporting characters like their sole purpose of existing is for the main character.
But they don't just go into a holding cell whenever they're not in a scene. They have lives. You don't have to write about it all, but you need to realize this. Your side characters still wake up in the morning, live out their days, and go to bed every night. The more you're aware of their daily behaviors, passions and time spent, the more interesting they'll become - both to write about and read about. They don't need to be the hero, they just need to think that they are.
So, take a break from your MC and let your writer imagination take over - following a day in the life of your side characters, and see how they behave. Really get to know them.
So, you now know what they do, even if it isn't information you'll choose to share in your story. Fantastic. The next thing you need to do is make them unique.
Everyone always talks about giving your MC something that makes them who they are - authentic and interesting; something that gives them a voice.
Why wouldn't you do that for your side characters as well?
I'm not saying that you need to give your side characters eye-patches and large scars. You don't need to give them superpowers, or make them an assassin, or have them part of a secret government agency (although you sure can). But you need to give them something that is interesting. Whether it be everyday or completely insane, side characters are still unique people - so show us that. Give them a passion, a hobby, something that is defining that readers will remember.
Have them interact with other characters, whether or not it's your MC, and display a little bit of that interesting personality, keeping their daily behavior in mind.
As mentioned before, the most important thing to remember about your side characters is that they're people with lives. Your MC is not the center of the universe, with all other characters revolving around her. Every character in your manuscript - from the psychopathic villain with a full monologue to the creepy pizza delivery guy with long hair and a bird tattoo who only says one line - has their own stakes, their own wants, their own goals, and their own obstacles getting in the way.
What, you think just because I'm not a title character means I only live when the superheroes need me? Of course not. I'm being sent on all kinds of missions, working with my team, occasionally having a little me-time once in a blue moon. But I never just disappear. (Okay, maybe I did one time, but you try dying and coming back. It's a process.)
Your side characters should be the same way. They all have their own story going on, and it just happens to intertwine with the story of your MC. Don't forget that. Don't overlook your side characters. Interesting supporting characters make for a more interesting read. Spend some time on the little guys.
And, who knows? You might enjoy a side character so much, you decide to do a spin-off just so you can have them as a lead.