When I do mystery sessions, I try to take a page from Agatha Christie. Everyone is suspicious, everyone has something to hide, everyone is a suspect.

Okay maybe not everyone, but certainly multiple suspects.

Often I think a big problem with D&D mysteries is that, to start with, the players have too little to go on. Somebody is dead/something bad happened, but the GM doesn't want the mystery to be easy to solve, so the bad guy covered their tracks waaay too well. Everyone has an alibi, none of the evidence is damning so the PCs just sit there with no paths to follow and the game durdles.

An alternative is to make it so that several suspects look likely from the beginning, and the adventure is about narrowing it down to which one of these highly suspicious people with a good motive and opportunity actually did the deed. Each red herring the players investigate really does have terrible secrets they are trying hard to hide, just not that they are the murderer.

This has several side benefits:
  • the suspect will act suspiciously by trying to hide their secret, increasing plausibility
  • finding out the suspect's secret is a pacing aid; while the players haven't solved the main mystery, there is still a sense of discovery and forward momentum
  • the GM can make the investigation process a little easier and rely less on stone walls to stretch things out, because the mini-revalations along the way to the main mystery won't end the session.
  • you can have some of the red herrings be willing to fight to protect their dark secrets, which breaks up the long investigation bits by putting small combat encounters throughout the session.

There are lots of other things you can do as a GM to reward player ingenuity and make them feel more like they are actually being clever in solving this mystery as opposed to on the rails, but structurally speaking this had made the biggest difference for me. Not going to lie, I'm still not a huge fan of RPG mysteries, but if you want to do one I think Agatha Christie has a lot to teach GMs about satisfying mystery structure.