Hi Linda. A bit of a disclaimer. I am not a full-fledged Pressbook guru like some users in this forum, but thought I would share my experience with formatting problems. The best way to figure out why the indents are missing is to examine the HTML code using the “text editor.”
Since you mentioned global changes do not fix the problem and you are a coding newbie, you may need to go through some trial-and-error troubleshooting because there are a number of things that could be causing the non-indented paragraphs. Try reading this Pressbooks guide to see if any of these issues might apply to your case, https://guide.pressbooks.com/chapter/paragraphs-indents-no-indents/.
For example, the indents may not be visible in the “text editor” or “web preview” prior to exporting the PDF file. Of course, this is not the source of the problem if you are viewing the exported PDF file in Adobe Reader and the indents are still missing!
If the non-indents are happening in both the Web and exported PDF, you may need to use the “text editor” to manually insert a paragraph tag at the beginning of the new paragraph (“p” surrounded by <> angle brackets, no spaces) and an end-paragraph tag ("/p" surrounded by <> angle brackets, no spaces) at the end of the paragraph. Below is an example. I added spaces around the “p” and “/p” so you can see the code.
< p>Here is the first sentence in a paragraph. Here is a second sentence. And another, longer third sentence, which we will make long enough so that we are sure it will wrap around to the next line.< /p>
Also, I have noticed that occasionally, formatting that appears fine in the Web version does not transition smoothly into the PDF or ebook files.
Sorry I could not give you a quick-and-easy answer. Formatting can be a finicky beast.