Why can't I add another widget to my custom dashboard?

Custom dashboards permit a maximum of eighteen (18) widgets per dashboard. You can view the number of widgets currently on each dashboard in the dashboard manager view, found by clicking the wrench icon at the top right of the page.

One possible workaround for viewing more data in a cohesive way is to strategically split the widgets into multiple dashboards and add all of the relevant dashboards to your carousel. Turning on the carousel will rotate through the dashboards automatically at a regular interval. This can also be accomplished in the dashboard manager.