Problem with epub display in generic reader

[Hoods7070]

Hi there - my first post so please be gentle, and assume I am an idiot. (I'm not, but let's pretend I am.)

I've successfully converted several (all bought from Amazon) AZW/mobi books to epub for use on a cheap Chinese reader. Three or four of these worked fine, ie perfectly readable, but I'm having grief from two.

One of these only shows "Start" in the TOC on the reader, and that opens an empty page, nothing further openable. This book was not encoded, BTW, so may be someone's self-published epub (??) - no idea as I have never used an ebook reader before. If I choose Force generation of TOC, I get the TOC, but clicking on chapters on the reader brings nothing up. Very frustrating - I've tried various things to no avail.

With the second, a Michael Connelly book, so presumably a publisher's AZW, my reader opens all the chapters fine, but there are no indents and no blank lines between paragraphs. (I asked Calibre to put a blank line between pars.)

I'd be really grateful for some help on how to set Calibre up for both problems. My feeble understanding of html coding doesn't help much.