Have your players complained? Although it's nice to have a scenario run multiple sessions, if everyone in your group is happy with resolving all the issues in a single night, then I wouldn't worry about it. Just call each adventure a minor milestone.
Complexity is the key. If you want your games to last more than one night, then you need to have a complex scenario. Make the players investigate the strange issues; get them moving throughout your city, talking to NPCs. Scatter the clues among various locations. Read a few mysteries (or watch them on film); the hero(s) never find everything out in the first few minutes or chapters. Use red herrings or sub-plots that work with the current main adventure.
Example, in our last sessions, the players had to find out why a crazed, feral supernatural was attacking people. By talking to the first victim (which they saved), they found out a storage vault of old books, etc. had been broken into and fenced to various parties. Some of the group went to the original vault site; others began researching whose books these were. The group found that a wealthy family had gone decadent and mad; the sole known survivor had died, instructing his butler to burn all of the family documents. Meanwhile, an illegitimate son (thought dead) ended up being cared for by the local Dominicans...until he broke out of his cell, ranting about recovering 'his stuff'. The break out occured as the vault was being burgled... Then the group was attacked by construct rats fueld by Outsider energy; it seems the family had made a bargain with Outsiders to protect themselves from being taken over by vampires. And the documents gave details.
In the end, the group confronted the warlock in a deserted park, protected by ghouls. The group aided the crazed survivor and managed to kill the warlock, recovering the book detailing the family pact with the Outsiders. One of the ghouls escaped (sans any documents).
This all took about three sessions, given the amount of interaction with NPCs (the abbot, the various criminal types, the WCV who was buying some of the documents as blackmail against another branch of the family; (the family had gathered a lot of info about who was trying to take them over); and a few battles (vs the thugs sent to intercept the document sale, and the construct rats).
It also left me with numerous plot threads to follow up when the time is right.
Hope this helps...
