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Hey, David Butler . . . you are probably the only person who has looked at this issue, so I want to thank you again for your input. I wanted to follow up about this. Since I am using Wordpress for the website, I used Wordpress's rewrite_rule function. After confirming that it was working correctly, I deleted the old sitemaps and created new ones, then  submitted them to Google. Shortly after this, the pages were successfully indexed. So from this experience I get the impression that using URL parameters doesn't necessary work -- I certainly am not going to use them in the future. I am going to mark this issue as resolved.