I have always used the "function" keys to adjust brightness on my ThinkPad X220 laptop under Xubuntu without issue. After upgrading to 16.04, they still work, but randomly stop working after about 50-75% of suspends. Suspend and wake again, sometimes they start working again.

When not working, they still cause the Xubuntu brightness screen bubble to appear and change values in the top-right of the screen, but the brightness doesn't actually change.

How do I troubleshoot/fix this issue?

Thank you for your help.