There are a couple of techniques I use whenever a character is being particularly stubborn in becoming "real" to me.
The first one I use when I have a plot in mind, but the characters aren't following. I call it the "connect the dots" or the "weaving" method. I'll use the example as I taught it to my 9yo son recently.
Me: "Ok, so your main character is a kid, right?"
9yo: "Yeah, and he's a spy."
Me: "Let's just focus on the fact that he's a kid for now. What can we automatically assume about any kid?"
9yo: "Um, he has a Mommy and Daddy?"
Me: "Well, kinda. We know that since he's a kid, he has someone that's taking care of him. Does your kid spy have someone that takes care of him?"
9yo: "Yeah, his grandma and grandpa."
Me: "Does he get along with them?"
9yo: "No. Not really. They made him be a spy."
Me (surprised): "Oh really? How did they make him be a spy?"
9yo: "They told him to."
Me: "Sounds like the kind of kid that would just obey whoever is in charge of him. That sounds like it could be really important. Does he like being a spy now?"
9yo: "Yeah. He thinks it's lots of fun. He does lots of running and jumping."
Me: "So he's very athletic. Was he always that way, y'know does he play any sports or did he just get into it because of spying?"
9yo: "He plays baseball too."
And so on, back and forth like this. Asking questions about "What kind of character would do this and why do they do what they do?" until something solid happens.
I'll then bring in the second technique when the first one just isn't quite working and I'm having difficulty making the character interesting. Sure, he or she sounds human/solid/whatever but there's just that "something" that isn't making that personality interesting enough to actually write about.
I'll use another example, this time from a long ago abandoned novel I called "The Game." The premise of this novel required that each of the characters be in top physical condition, so that was a given. Since this was written in the 80s and famous female athletes weren't as common as they are now, creating my MC was a piece of cake. Her nearly as important teenaged brother, on the other hand, was quite a bit more difficult. All I knew was that he was a skateboarder and that his parents were the same as hers. I also knew that he got along with the MC and that their motives for joining "The Game" were similar (prize money...what else?) but he still refused to come to life. I gave him a skateboard for his athleticism which made him long and lean
Next, I started "putting" other stuff in his hands. First, common everyday stuff: pen, cup, soda can just to see if how he held them would give me any clues to his character. Next, other hobby-based items. A football didn't quite fit, a surfboard did, but that didn't tell me anything I didn't already know. Knitting needles he held as if it were diseased, but a violin? That one surprised me. He held it as if it belonged there. Sure, he wasn't a serious player and would certainly not get into Julliard with his skills, but this gave me a key not only into the depth of his character, but other insights as well. Such as the arguments he would have with his sister and how he may not be able to hear his parents calling him down for supper.
The trick with this technique is to not go so far that you start doing "reverse cliches" such as "hooker with a heart of gold" or the like, but to add something unexpected to an otherwise stereotypical character such as "skater kid", "working mom" or the like.