I think you've spotted the key symptom: "wether I select the check box "Taxable" doesn't seem to impact or affect the line items Taxable check box in invoice layout."
The taxable field in line items uses a looked up value field option to copy the value of Taxable from Products for each new record. Editing the taxable field in Products will not affect existing line item records. So if new line items records are not copying this field over or are copying the wrong value, then your sales tax calculation will fail to compute sales tax.
Have you made any design changes to your database that might have affected this?
Check field options for the taxable field in LineItems and make sure it has a valid looked up value setting. Make sure you haven't changed the value entered in the taxable field in Products unless you've checked and made any corresponding updates to the taxable amount field in Line Items.
In sample invoice ... how do you have the "taxable" box alway checked as a default? Have every entry taxable? thanks
Phils answer is excellent and I might add my comment.
You should also check the inspector to see if the settings for that check box are correct. Here are my settings for that check box:
Display Data From: Line Items Invoice::Taxable
Control Style: Checkbot set
Values from: Boolean - "1"
Feild entry: Browse Mode checked, Select entire contents on entry checked
Go to next object - all selected except last one for spelling
Data Formatting = number - as entered