Your host or other plugin may have disabled compression by setting the WPSC_DISABLE_COMPRESSION constant. That silently disables compression. You could search for that constant to see if it’s set.
If you have compression enabled in PHP then it will unset itself too, but that will show you a warning.
Thanks, I still work on that problem! (I work on the website only every now and then) I realise that no matter what plugin I use it won’t work. I tested all or the better ones. And my c-panel shows that g-zip is enabled. Where the heck do I find WPSC_DISABLE_COMPRESSION?
You’ll need to ssh to your server and run something like grep -r WPSC_DISABLE_COMPRESSION * in the directory where WordPress and your website is installed. That will find where that constant is defined.
You could also ask your host support if they have done this. They might be able to track down the problem more easily.