Thanks for the help!
I think I figured out what happened, though I think the behavior is still odd. I entered the tag I wanted on the search bar as tag:X, where X was the tag I wanted. I did not hit enter, however. Immediately underneath was a list (I suppose) of what EN though I was searching for. One of these was tag X. I clicked on it only to get one note. I knew that was incorrect because I had multiple notes tag as X. I redid the search, but this time after entering tag:X in the search bar I hit enter, and EN found all my notes tagged with X.
This seems to me to be strange behavior. Why would EN list tag X as a possible search parameter when clicking on it does not return all the notes tagged with X?
Thanks. I contacted Readwise and was told that they had made a decision not to export Readwise highlight tags to Evernote tags based upon user feedback. According to Readwise users though exporting Readwise highlight tags would "clutter" up the list of tags in Evernote. For me, at least, I would like to have them exported over.
Thanks for your response!