The allegory of the olive trees always seems so long and involved to me when I read it. Sometimes I read it and get a lot of insight from it. Other times I struggle with it. This is one of the times that I was struggling to pull meaning from it.
But what I did get is that sometimes you can plant a seed in the best soil and nurture it carefully and have all the best expectations, and it still brings forth bad fruit. Other times a plant grown in bad soil with little care can unexpectedly flourish. So it's important not to get set in your expectations of others just because of where they came from or what their past history may be.
Also, with enough care and nourishment, sometimes a plant that brings forth abd fruit can be turned around and eventually have good fruit again. Reason enough to have hope and keep working toward that end.