Hello,
When adding a new dial, clicking existing will replace it.
You need to click an empty one.
If you don't have any more free space, then resize your group or create a new one.
To resize or change layout of existing one, right click the group name (or open the main menu) and select "Edit group". There you can change layout or dimensions to add more dials.
I hope this helps :)

Best regards,
Juraj Mäsiar