There are known issues with the current version of Jing that relate to multiple monitor support. Issues may occur when display properties within Windows are modified while the Jing is running. The capture process in Jing will not behave properly when Jing attempts to capture the screen (unable to capture secondary display, wrong pixel dimensions, wrong pixel coordinates).
This issue will be addressed in the next update to the software. Until the issue is resolved, closing Jing and reopening the program should allow Jing to capture from multiple monitors.