Conversion's structure detection working only sometimes

Hi! So I downloaded some webnovels in txt format, and I am trying to convert them into EPUB so I can read them on my tablet on-the-go. However I am facing some issues with the built-in converter, as sometimes the conversion happens smoothly (everything goes as expected, the chapter structure is detected, the page breaks are added in between each chapter and the formatting is done correctly) as as seen in figure 1, but other times, even if the source .txt file is formatted exactly the same as the first one, the structure detection does not work so there is no ToC created, no chapters are detected, no page breaks, nothing, only a long text as in figure 2.

Why might this happen?? I have done so far everything I have thought about, but nothing comes to mind and I am still new to Calibre anyways, so maybe there are things I don't know I can do.

To give some more information on how strange this is, both .txt files that were the source for Fig1 and Fig2 were formatted exactly the same, with each chapter title marked between the <h2>...</h2> tags, with the same amount of spacing and empty lines, everything is the same, but still some of the files work and some not. It cannot be a problem with the XPath expression because I selected a bulk conversion of 27 Webnovels (all identically formatted), and leaving the XPath as default the results where the following

• 12 successfully converted with chapters, page breaks, etc.
• 12 not successfull, no chapters nor page breaks, just text
• 3 didn't convert, there is no text, only 1 page with the cover art and that is it

So, any ideas what could be happening?? What am I missing, what is wrong with my Calibre or my .txt files??

Attachment 195673
Figure 1 - Working as intended.
Here is the log for this book conversion.
[Attachment 195674
Figure 2 - Not working
Here is the code for this book conversion.
Attached are 3 .txt files, one for the example on Fig1, second for example on Fig2, third is an example of one of those 3 that didn't convert, where the result was only a cover page and that is it. Also attached are the epub files obtained, so that you see exactly what I am getting.

Thank you for your help on this!

PD: The txt files have been trimmed at the end to ensure no more than 1Mb, but I think the size should not matter here.

Attached Thumbnails

   

Attached Files

	Figure1.epub (2.80 MB)
	Figure1.txt (962.6 KB)
	Figure2.epub (333.0 KB)
	Figure2.txt (939.6 KB)
	Problem3.epub (14.7 KB)
	Problem3.txt (865.6 KB)