Sunpak's main business is the planning, development, production and sale of aftermarket electronic flash units. The company also markets general underwater imaging equipment.
Some Sunpak flash units have an auto power save function that may put the unit to sleep after about some time of inactivity (from 5 to 15 minutes, depending on model). When using your flash as a wireless slave, the power save mode may stop the flash from responding after a while. If the manual does not tell you how to disable automatic power save, it may work to hold down the mode and select (SEL) buttons as you power it on, or to hold down the zoom button as you power it on.