See "Help" -> "Example Graphs" -> "Layout" -> "familytree.graphmlz" for a family tree example. Two things are important:
Nodes representing individuals (i.e. men or women) are not connected directly, but to a "family node". For each family node, there should be an edge from the father to the family node, an edge from the mother to the family node, and for each child an edge from the family node to the child. The direction of the edges is important!
Node colors are used to differentiate between different types of nodes. I.e. all nodes representing men have to be the same color, all nodes representing women have to be the same color, and all nodes representing "family ties" have to be the same color.
That said, without knowing your spreadsheet, the Excel Import settings, and the resulting diagram, it is impossible to provide advice on how to fix the problem. If you upload a screenshot of your Excel Import settings as well as a GraphML file for the resulting diagram, I will be happy to try to provide further advice.
Please see How to upload files to yEd Q&A? for instructions on uploading files here.