In the best case, the usage of minify optimization is a trial and
error process, it's not an "instant on" or "set it and forget it"
optimization technique.
There are lots of reasons why minify cannot work for all sites under
all circumstances and they have nothing to do with W3 Total Cache:
Your site's content, your server(s), your plugins and your theme
are all unique, that means that minify cannot automatically work for everyone.
What is minification exactly?
Minification is a process of reducing the file size to improve user experience
and it requires testing in order to get it right — as such it doesn't work for everyone.
The interactions and dependencies of CSS or JS on each
other can be complex. Themes and plugins are typically created by various developers
and can be combined in millions of combinations. As a result, W3 Total Cache cannot
take all of those nuances into account, it just does the operation and
let's you tune to what degree it does it, it doesn't "validate" the result
or know if it's good or bad; a human must do that.
Still want to get started? Now for the Pro' tips:
Start with minify for your CSS using auto mode first.
If you have any issues at that step, contact your developer(s) and report a bug. They should be able to
point you in the right direction or correct the issue in a future update.
Once CSS is optimized, try JS minification. If auto
mode doesn't work for you, be sure to check the web browsers
error console to quickly confirm that the optimization isn't
working. If the JavaScript is working, you can either make
additional optimizations for user experience like experimenting
with embed locations etc or further reducing file
size etc. However, if you're having errors try
the "combine only" option and if that still generates errors,
there are bugs in the code of your theme or plugins or both that
prevent minification of JS from working automatically.