There are two YouTube videos - one is handled successfully by ochFitvid (the latter one) but the first one is not.
The iframe of the one being handled successfully is surrounded by <p></p> tags.
However, the other one is surrounded by <p style="text-align: justify;"></p> tags. I have tested <p style="text-align: left;">, <p style="text-align: center;">, and <p style="text-align: right;"> and those also prevent the video from being "FitVidded."
I can do a find and replace of all those tags appearing before iframe tags with various video urls in them, but it will be tricky to get bloggers to make sure this is not happening in the code as they work.
uploaded a test version in the OP (your first post in this topic).
This version 'widens' the search to not only include <p>...</p> but also all variants where styling is set on the <p> element.