Is your data in a Geometric Network? If not, then you have more complex configuration to do. If so, then when you did the initial configuration of the dataset, did you import from a map document? Did you uncheck any of the geometric network features?
The rule is that a schematic link must have end nodes. We need all the connectivity. So if you don't import everything from the geometric network, you will surely have cases where one of the end nodes is not configured and therefore the link can't be displayed. So again, you need everything, including the net junctions. Then if there are things you don't want on the diagrams, you configure Rules to reduce the nodes.
Not sure what version you are on as it isn't indicated, but here is a 'quick start' posting for 10 (it is similar pre-10 but with different terminology):
http://forums.arcgis.com/threads/1079-Beta-10-Schematics-Quick-Start-for-Utilities?highlight=schemat...