Symbolizing using graduated colors in ArcgisPro does not return an error when the maximum sample size is reached. It's really difficult to troubleshoot missing contours when there are no error messages.
This has been marked by Kory Kramer as a Duplicate of https://community.esri.com/ideas/14414-no-sample-sized-reached-error-in-arcgis-pro. Please vote and comment there. Thank you.