Redirect Only Works After 404 Generated

I’ve used this plugin in the past and it is great. However, I’m having an odd issue here.

+ I imported about 500 redirects in.
+ I then started testing out the redirect by copying in the “source URL” into the browser.
+ Unfortunately, it goes to a 404 page.
+ I then check the 404 list within the Redirection plugin.
+ I find my page there and hover over it so I can click on the “Add Redirect” link.
+ I add the redirect again this way.
+ When I test it, it works!

I have copied and pasted both source URLs — the one from the import and the one I manually added — into a file so I can compare them… and they are the same! Why would this be? Is there anything else that could be causing this to not work?

Thanks.

https://wordpress.org/plugins/redirection/