There are two different criteria in Smart Groups: "Username" and "Local User Account." It seems like the OP was referring to "Username."
I've tried every permutation of actual usernames (as listed in the device records) I can think of, but no devices are ever included in that Smart Group.
Seems that either "Username" isn't working or that the OP and I aren't understanding what the "Username" criteria means.
Anyone had any luck with using the "Username" criteria in Smart Groups?