Laptop sometimes continues drawing power while the lid is closed

Recently I updated my System76 laptop from Ubuntu 14.04 to 16.04. Afterwards, occasionally but not always, when I close the lid, the battery appears to continue to drain as if the computer were still active.

For example, I recently went to sleep with my laptop fully charged but unplugged, and when I awoke, the battery was completely dead. On a couple of occasions, I've left the laptop closed and unplugged for an hour or two, and then when I pick it up, the bottom feels warm.

Twice recently, when I've noticed this behavior, I've tried to shutdown or restart the laptop, and the Ubuntu shutdown screen with the cycling red and white dots has remained showing for a minute or two until I lose patience and do a hard power cycle. I'm not positive there's a connection, but that rarely if ever happened before.

I've tried some approaches suggested by answers on this site (here, here, and here), specifically:

• added a line HandleLidSwitchDocked=suspend to /etc/systemd/logind.conf
• added a line HandleLidSwitch=suspend to the same file
• installed the package laptop-mode-tools

...but to no apparent effect.

Any tips for resolving this issue?