The employee may have printed to a secure queue when they thought they were printing to a non-secure one. They will be waiting for the job to print out, when in fact they need to release it from a terminal.
If the terminal has been improperly configured, so that it is associated with a device other than the one it is attached to, then employees will not see the job print out at the device. The job will probably have printed, but at a device somewhere else. Use Blueprint Administrator to check that the terminal the employee is using is associated with the device they are releasing jobs to.
If the employee is printing to a Print Group, it may be that the device the job was released to uses a different Printer Description Language (PDL) from the queue that it was initially printed to, meaning that the device cannot print the job. It is a requirement that all devices in a Print Group must use compatible PDLs - check the drivers of the devices in the Print Group that the employee is printing to to ensure that they are all compatible.