I have seen the same thing - where the left margin is too big. It is a bit of work to fix, but pretty easy. And thankfully it doesn't happen that often - so far.
See this thread:
https://www.mobileread.com/forums/sho...bi+left+margin
More specifically this post:
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" Fixing this problem for Dummies:
If anyone wanted to know the quick step by step I am doing to solve the problem I'll describe it below.
-First go to Calibre and convert Epub to Epub (yes the same format), making sure to go to "Page Setup" and change the left margin to 0.
-Then go into Sigil and open the new Epub file (it should automatically update or save in a new location). If it doesn't open or has an error it might be the old file you are opening, not the new one.
-Go under the folder "Styles" under the Book Browser, and open the Stylesheet.css
-In the style sheet, all I do is look for every single thing that says marginleft OR marginright and make sure the value is 0 for everything. If it says 1em I change it to 0em. If it says 5pt, I change it to 0pt. If its 0 I leave it at 0. If it says auto I leave it saying auto.
-Control S (Command S for mac) to save. Then exit Sigil
-Back in Calibre all I do is click convert to Mobi this time to make a Kindle compatible file format. Again make sure under "Page setup" it has 0 for the left margin value (it should if you did this before).
-Now when you put the file on the device it should be in a nice normal format. Everytime i've followed this procedure it has worked very nicely. I DO wish that Calibre could do this by itself, as it takes like a minute per book conversion, but its not the worst thing in the world.
Since I'm somewhat computer illiterate this HOWTO would have at least saved ME a ton of time figuring out how to fix this problem when I first had it."
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