is the tax code 'used in all journals'

Yes   :-/   "All journals" for all options

Is the linked account showing under 'taxes' correct?

Yes...   This is so frustrating !

has it ever worked ?

I wonder if you temporary change 'all journals' to 'purchase', save settings, go back and change to 'all journals'

Yes, I've been with this company 6 months and never had this problem.    

and yes, tired changing them back and forth.     The boss even had this bright idea to restore from a couple days prior, which only resulted in me having to re-do some work :-/

ok, so I'm looking at "assign Tax codes"    selected customers (blue bullet) and  the menu for  "with this default tax code"  only shows no tax,   and most of my customers (and most frequent) are on that list.

You have uncovered one possible reason for your GST not showing up. You can change the tax code to show the tax code that would most often be used by the vendor/customer. Another occurs if you are using inventory/services and each item is coded as No Tax.

Both of these items can be over-written on the Sales Invoice or Purchase Invoice by changing the code to the proper one in the Tax column. Use the drop down menu to pick the correct code.