When to use single/parallel/quad stations?

As it currently stands, the way I’m trying to do it is use single stations for low demand areas to connect places to a more centralized quad station, then using parallel stations for high demand loops from the central quad station.

It doesn’t work the best, and often I’m notified that the routes I’m trying to set up to connect single stations to my main quad stations don’t have a possible connection. Any help?