There’s a chance you have not enough RAM, causing lots of swapping, which means the CPU won’t be pegged because the machine is always paging things in and out.
You can measure things from the command line:
cat /etc/lsb-release
df -h /
free
vmstat 5 5
either with the machine in a quiet state, or at the time that it’s stuck. (Connect from a second window to get a second command line. Or similar.)