Check the settings under Setup | Settings | Company | Sales Taxes:

- The Tax Codes connected to a tax (click on the Code to see the Tax Code Details screen)

- The Taxes have to be connected to the proper G/L accounts.

- The Item and Customer have to be set to allow (not exempt) the tax code.

If there's a disconnect anywhere along there, then the sales tax won't calculate.

Three places to look first.

Inventory:  If you are using inventory/service items, check the ledger for that item and make sure the item is not marked as tax exempt on the Taxes tab of the Inventory and Services ledger (where you set up the item).

Customer: Customer may be marked HST exempt but not GST exempt.  Check the ledger where you set up the customer name and address and look at the Taxes tab to see if the customer is marked exempt.

Tax setup: Under Setup, Settings, Company, Sales Taxes, Taxes - make sure the tax is not marked Exempt

Setup, Settings, Company, Sales Taxes, Tax Codes, then double-click the HST code and make sure the proper setup is done including the percentage under the Rate column and the status is taxable.