This got too long for a comment so I'm reblogging😅
If the mystery's too obvious than I'd start by:
1. Removing clues (and hints, foreshadowing etc). Make a list of all clues and eliminate any you find unnecessary. Be extra critical of early clues that may reveal too much. If that's not enough, or they are all necessary then-
2. Start obscuring the clues you have. Make reliable info unreliable and discredit witnesses. A character saw something suspicious, but now she's drunk or it's dark & rainy or the window was foggy or she had a head injury.
First hand testimony could change to second or third hand gossip.
Introduce conflicting information.
Distract the reader & character from focusing on the clue- make it a side note to the action occurring in the scene or there's something more suspicious happening to grab attention or it's hidden in a list of other things.
These are all things I usually do when I go back through the story, I personally can't do it until I clearly know the ending.
As for plot twists, I'm not very useful. Any I've done have just come up when I'm writing. But I keep Brandon Sanderson's advice in mind when deciding to use a plot twist idea, "always give the reader MORE than they expected in a plot twist to be satisfying."
(I'm about to reference The Sixth Sense for its famous plot twist, apologies if this is a spoiler🙏)
The story sets up that a kid is seeing and talking to ghosts. The writer meets those expectations and fulfills the promise he set up- a kid is really seeing ghosts.
But the writer gives even MORE by revealing that the other main character is also a ghost, who didn't realize he was dead. This is bigger than the audience expected.
If the kid suddenly woke up and realized it was all a dream, that would still be a plot twist, but not a good one because it would give the reader LESS than they expected. They didn't get anything that was promised earlier.
So maybe think about what a predictable ending would be based off the story so far- where the reader expects the story to be going- then brainstorm what can be added that would make the situation bigger or more extreme.
I hope this is helpful. While my stories contain mysteries, it's not the primary genre and I have to do a lot of back and forth on those elements until I'm happy with it.