If your latest revisions don't appear when you send a test email, the most likely reason is that you didn't save your draft before sending.
Test emails are sent based on the last saved draft.
If you haven't published your post yet
The best way to ensure your test email always contains your most recent changes is to save draft before sending a test email.
Why doesn't it automatically send my most recent changes?
Newsletter Glue can only send content that's been saved.
If you send a test email before WordPress has autosaved your post, then your test email won't include your most recent changes.
If you notice your test email is missing your latest changes, simply save draft and send your test email again.
If you have already published your post
Don't click Send newsletter again until you're ready to send a new post when you click Update.
Instead, make all your changes first, click Update, and only then click Send newsletter again. From there, you will be able to send a test post with your latest changes.
Then, the next time you click Update, your post will immediately be sent out.