This genre has always intrigued from the time I read 'And Than There Were None'. I thought it was a great mystery novel, but I've read other books from the mystery shelves that aren't so great. Some plot lines are too complex and you really don't care about the who dunnit as you get closer to the end. I've tried writing a couple of mystery short stories, but I can't do it properly. Either the stories I write are too obvious. Or so complex the person I've had read them is left scratching their head. Any pointers other than study more novels?
I don't have any particularly great advice other than read more mystery novels and maybe watch some classic mystery movies. Maybe find some masters (like Agatha Christie, Arthur Conan Doyle, Raymond Chandler, and contemporaries like Harlan Coben etc - the list goes on) and try to note their plot lines down to find similarities, patterns or differences, study the way they write the twists and turns and red herrings etc? I'm attempting my first mystery at the moment and I know the beginning and the basic ending but what happens in the middle is anyone's guess! I haven't even decided on the murderer yet (it's between three characters) as I am waiting to see how they evolve. However a friend of mine plots every little twist and turn in her mysteries, before she begins, and lets the plot drive the story. I don't know how she finds her story lines; she seems to have a head for puzzles and it's almost like watching a jigsaw fall into place as she understands what is going to happen and with whom. Her brain must be a very interesting place....
Study more novels, and keep practicing. You can't learn writing without making attempts and analyzing how it coule be better, and you can't analyze effectively without studying how other make it work (and how some others fail to make it work). Not every published piece is successful, but the more you read, the more readily you will spot the failures and understand what is broken.