Not finding Aunt Dimity's Death by Nancy Atherton is due to the method the plugin uses to check for books. Checking search.overdrive.com for books by Nancy Atherton returns 47 results as shown in your first search log. Aunt Dimity's Death is not listed there even though it is clearly available at libraries using Overdrive. I have seen this before with other books and I suspect that Aunt Dimity's Death will eventually show up on search.overdrive.com. It appears that the index there is only updated periodically. Trying to make the plugin work with the search functions of individual libraries would be difficult so this is going to continue to be a limitation of the plugin for now.
The problem with books by Agatha Christie (and other prolific authors) is that search.overdrive.com is limited to 501 results, causing some available books to be missed. The plugin was written to search by author and then check the returned results for specific titles because this is generally faster and also because I had discovered that searching Overdrive by both author and title sometimes fails to return books that are available. I will update the plugin to search for books by both title and author in cases where the search only by author returns too many results. That should fix this problem.
The failure to locate the book by Maria V. Snyder is due to a bug that I will fix in the plugin. The author search at search.overdrive.com is sensitive to word order. A search for books by “Maria V Snyder” returns 102 results, but a search for “Maria Snyder” or “Snyder Maria V” returns no results. The plugin attempts to correct the word order in cases where there is a comma in the name, but a bug causes it to fail on some names. I will correct this in the next release.