The most likely cause would be a gap in the path. Check to see if the path is closed, or double-click on the Shape Builder Tool in the Toolbar to bring up the Tool options. See if Gap Detection is active, and if so try adjusting to a greater gap length.
Thanks for the reply.
I tried Gap Detection with all sizes. Also tried the other various options within the shape builder tool. Still having the same problem.
Is it possible for you to share the document?
Thanks for sharing that. The problem seems to be that the large construction is a Compound Shape, and the Shape Builder Tool requires simple paths. Go to either the Object menu > Expand Appearance or to the Pathfinder panel and click Expand. The Pathfinder panel flyout menu also has Expand Compound Shape. That should allow the behavior you're expecting.
That did the trick. Thanks for the assist Peter!
Excellent! You're welcome. I'm glad I could help.
Expand appearance is greyed out
Could you please start a new thread and provide details about your problem? This thread is marked as having a correct answer, and we have no way of knowing whether or not your question is even remotely similar to the original post.