Three steps of a live campaign sent blank emails because I updated them with plain text and line breaks. The tool dropped every word and said success.
Match the format of the steps that already work. For mine, that's one paragraph tag per paragraph, with no bare text.
After any change, count the visible words in each step. Strip the tags first. "Not empty" isn't enough: eight line-break tags is 48 characters.
Check merge fields exactly. {{firstName}} works in my tool; {{first_name}} prints nothing, so the email opens "Hey ,".
Send yourself a test of every step you changed.
Check the delays. In my tool a step's delay is the wait after it, so the last step is 0.