Hi Mikef,
On the stock screen Save & Email becomes visible and replaces the Save & New button when the Email notification when saving? box is selected. It remains hidden if it is not selected. This is done with the actions in the screenshot below:
Does Save & New appear on the screen?
If Save & New is visible it may indicate a problem with the actions on the Email Notification when saving? box.
Below is what usually happens:
If this is an issue with the stock invoice screen you may want to call support - 1-888-925-6238 x2.
I hope this helps.
-Cian