Hummingbird minification problem

Hi, I installed Hummingbird yesterday, went through all of the settings and improved performance significantly.

However I also activated minification and when I looked at my pages today all of the font styles for H1, h2, h3, h4 and body were nor working properly. These are set in the CSS customisation section of my Divi theme.

So I unset minification and they all started working again.

Just so you know, I did not change any of the default settings that minification creates when you activate it, so it was not something that I did wrong. (i.e. I did not move anything from header to footer or include or exclude anything)

Could you please explain why this problem might have occurred and whether there is any more detailed tutorials of the use of minification, as there is scant information in the settings part of the plugin.