Up to the screener; and yes, if the screener believes a passenger is attempting to do something to defeat the WTMD, they can ask the passenger to re-submit themselves for WTMD screening and if that person refuses send them for secondary screening. It is not retaliation; it is a legitimate security concern.

Many factors play a part in whether or not the WTMD alarms, even the speed at which you walk through it may prevent an alarm.