Q: iPod, iPad and iPhone all appear in list of devices in Find my iPhone, but even when all switched on, no location available for iPad and iPod. Have checked everything suggested. How do I resolve, please

Have iPod, iPad and iPhone linked and they all appear as my devices in 'Find My iPhone' app, but only iPhone is located on map. 'No Location available' is displayed for both iPod and iPad. I have checked everything is correctly set up. Has anyone got any idea what might be wrong,please?