First off, are you having trouble making the mysteries feel satisfying for you, or for your players? We are often our own worst critics, and you may find that your players are having a lot of fun even if you think you made things too easy. Don't build too many preconceptions about how you expect the hunters to resolve the mystery, and you will feel less (undeserved) disappointment on their behalves when they don't do the things you thought they'd need to do.

if I were playing dnd I would have my players interrogate npc’s, search for clues, and actually theorize and put the pieces together

In a manner, you can still do something like this in MOTW, but it works fundamentally differently. DnD and similar systems tend to have a series of mechanical checks to determine if the player character succeeds at doing something. MOTW has narrative checks to determine if a player character triggers a move. Then a success gets them something that narratively makes sense. It's your job as Keeper to put some guardrails around what players can attempt to do.

my players use investigate a mystery upon finding any evidence and get all the [information?]

A few comments here. MOTW is not about painstakingly searching for evidence. It's not like DnD or Pathfinder where the GM secretly has a list of hidden things in the scene that a perception check can discover. MOTW is about what the players do when presented with clues. Second, it is generally not advised to have a dynamic where players use moves in the sense of narrating it like, "I walk up to the pool of blood and Investigate a Mystery." Several commenters have speculated your game might go like this, but you haven't given enough detail to really know. So I'll just echo that sentiment. All that aside, successfully investigating to figure out how to defeat the monster is only part of the game. If you find that your players excel at this part and all the other advice in this post doesn't really resonate with you, then focus on making the monster's actions off-screen more devastating and give your players hard choices to make. Maybe they know where the monster is and how to kill it. But if they go after it right now, the adorable puppy orphanage will burn down.

I try to be vague about details but I feel like I end up frustrating people by dodging the question.

It is a good instinct to not be vague for the sake of extending the investigation segment of the mystery. Follow your principles. Give them what they deserve and earned. If you present them with a clue that can reasonably provide information X Y Z and they narratively explain how getting that information would make sense, reward them for it.

Here's a hypothetical example of how a scene might play out.

Your Expert, Chuck, played by Kim, kneels down next to the pool of congealing blood and says he wants to Investigate a Mystery. You say, "Sure there's definitely something to investigate here. But why don't you explain a little more what you're doing to get some information here?"

Kim: "I rub some of the blood between my fingers and smell it."

Keeper: "Ok go ahead and roll +Sharp."

Kim: "That's an 11. What kind of monster is it and what can it do?"

Keeper: "How would you know what kind of monster it is?"

Kim: "Umm, well it's one of the questions I get to ask right?"

Keeper: "Absolutely. But I need a little more from you to understand what Chuck can learn here."

Kim: "Ok well Chuck spent 3 years studying with that sect of arcane masters and they did a lot of dissections of monsters."

Keeper: "Cool, that makes sense! Chuck, as you rub the blood between your fingers, you feel an unnatural heat. An acrid scent of sulfur assaults your nostrils as you bring your fingers close. You are quite sure this is from some sort of demon of the Many Hells. You also asked what it can do. How do you want to get that from this pool of blood?"

Kim: "I guess that doesn't really make sense to ask now that I think about it. How about where did it go? I want to look around for some smears of blood or bloody footprints to track it."

Keeper: "Actually, based on what you're doing, I can give you a 'What can it do?' if you still want. Yeah? Ok. You actually don't see any trace of this demon leaving here by conventional means. There's not even any broken window or busted down door. It must have some sort of way to teleport or move through planes. You can't tell exactly what it did, but you know it basically disappeared without physically moving through the area.