I'm the DM for a party of 3 people. I made a detective campaign where they're trying to uncover a conspiracy that is fake in the first place.
I've prepared a lot of different ways for them to find clues, with multiple contingencies in case they miss something, and so far they've been able to find and figure out literally everything, even the difficult stuff. I prepared letters, a clue board, a necklace with a hidden piece of paper inside... I really put a lot of effort into this.
They noticed that the handwriting on the letters they're looking for was different on two of them.
(I asked different people to write different letters IRL for immersion, and they actually noticed the difference)
They didn't like the name of one of the people involved, so they searched through the city's chronicles to check whether someone like that actually existed
(Thankfully, I had those prepared)
They literally turned into a dog so they could identify the scents of different people and crime scenes
They noticed inconsistencies in the accounting books
(I made several pages of them specifically so they could get lost in all the numbers)
I know they've discovered secrets about some of the noble families, and they haven't told anyone or confronted anyone because "the time isn't right yet"
They're even hiding some things from me, which is starting to make me paranoid
They have no problem with the campaign and they've assured me that they're genuinely having fun. The campaign isn't over yet, but I honestly don't think I've prepared anything harder than what they've already solved
Do you guys have any advice, methods, or puzzles I could use to make the investigation more complicated?
Edit: I don't want them to lose, I'm not playing against them and as I mentioned, they're happy with the campaign and having fun. I just feel like I haven't given them enough of a challenge