[ 92.192894] PM: Syncing filesystems ... done.
[ 92.268839] PM: Preparing system for mem sleep
[ 92.268847] Freezing user space processes ... (elapsed 4.87 seconds) done.
[ 97.140091] Freezing remaining freezable tasks ... (elapsed 0.00 seconds) done.
[ 97.140218] PM: Entering mem sleep
[ 97.140228] Suspending console(s) (use no_console_suspend to debug)
[ 97.140386] pm_op(): usb_dev_suspend+0x0/0x20 returns -2
[ 97.140388] PM: Device usb8 failed to suspend: error -2
[ 97.140390] PM: Some devices failed to suspend
[ 97.140421] PM: resume of devices complete after 0.029 msecs
[ 97.690227] PM: resume devices took 0.550 seconds
[ 97.690247] PM: Finishing wakeup.
[ 97.690250] Restarting tasks ... done.
The motherboard has a usb3 hub in it -- thanks to this post I figured out it was the xhci module that was holding up the suspend. I did 'sudo modprobe -r xhci' and confirmed I could suspend.
To "solve" it permanently, I'll put a file in /etc/pm/config.d with the line: