A couple things could be happening here. I don't have enough details to know exactly what you mean, but I'll try to guess.
First, remember that SGraphs are immutable, so if you did something like "addedges" or "addvertices" and expected the graph to change, it didn't, instead returning a new graph with the new vertices or edges. If you call "show" on the new, returned graph, you'll see the new data.
Second, if you are reassigning your graph with the new graph, with something like this:
g = g.add_vertices(...)
then you'll have to hit your browser's refresh button to get the graph to update within Canvas.
Let me know if that helps you. If not, you'll have to provide the exact steps that lead to the unexpected behavior.