If your clinic's GST status has changed from Not GST registered, to GST registered - you can make this change from Business Setup -> Your Clinic -> You will see at the top of the page your business details and you can change your GST status from the dropdown menu -> Select the appreciate status -> SAVE.

Once you have amended, the GST status will filter through to invoices. If you are GST registered, patients will see a GST total charge on the invoice:

If your GST amount is not totalling correctly, this either means:

1) The item isn't GST inclusive and will not show a GST amount.

2) You have not set the GST status correctly for the item.

If the status has not been set correctly, you can amend this by going to Inventory -> Items -> Search for the item you need to edit -> Click the blue magnifying glass button to edit -> On the right hand side under Tax Settings, select if the item is GST or GST free -> SAVE:

If an item you would like to edit has this field greyed out, this means the item is coming from a product Feed and cannot be edited. If you would like to make changes to a default product, you can select to clone the item.