Can no longer create metrics for previous data source
I'm encountering some odd behaviour in my report.
I added Google AdWords as a data source to my report, and then created tables first, as well as a new metric in the process.
I then added Google Analytics as a data source to my report, and created tables for Google Analytics.
I would now like to add a Google AdWords metric, and amend my previous Google AdWords tables. However, the 'create a metric' button is no longer visible. Why is this? Is this just a glitch?