When I write mysteries there's a few things I make sure I do.
1) Balance the level of suspense carefully: In order to keep readers interested, you have to build suspense with cliff-hangers or new clues. However, be careful not to stretch out the suspense, or your reader can become bored or frustrated.
2) Don't give away the big surprise early! Always lead up to what the big mystery really is with hints and clues. It's okay to leak some important information about the subject occasionally, (to help keep the reader's attention), but don't give away too much too soon.
3) Always keep the reader guessing and thinking. Have a character ask a question to themselves. The reader will wonder about the answer too. Also, give some false clues once in a while to slightly confuse the reader and make them rethink what the mystery truly is.
That's all the advice I have. I hoped it helped.