Linking Plot and Subplot

How closely must subplots mirror the main plot? In STEEPED IN MURDER, the subplot was the thing pushing my amateur detective, Abigail, to investigate the crime. But when you're writing a series, it gets a little obvious when you play that trick over and over. There are other ways, however, that you can connect subplot to the plot. And it IS important that there be some connection, so the subplot isn't just hanging out there. As humans we LIKE to see connection!