Tested in:
Chrome Version 36.0.1985.143
and
Safari Version 7.0.6 (9537.78.2)

The Create Issue Icon in Plan Mode in JIRA Agile located at the bottom of the screen cannot be clicked due to placement. Cannot scroll to see it if the Board goes past the end of the screen. Icon Is cut off (see screenshot)

As a Workaround:
Use the Browser Zoom Out function until the option is visible if the board is not too long, and the option becomes available
after clicking the option you can zoom back in and scroll to bottom to use inline mode if the Scrum Board filter is set to a single project. or a popup will come up if the filter is set to multiple projects.