Flight IDs can take some time to appear. On these non-ADS/B aircraft, a ModeS ground station has to request the information before you see it in Radarbox. So, if there's no ground station nearby, you have to wait until the aircraft is high enough before a more distant ModeS ground station requests this squitter. I think!!
Bear in mind, though, that some non-ADS-B aircraft (such as USAF C-17s) still transmit the Aircraft Identification Squitter containing the callsign at 5-second intervals whether or not they are being interrogated by a ground station.
In SBS-land, these get shown by a solid circle on the display, meaning "
we are receiving ADS-B messages from the plane, but that they do not contain latitude and longitude data". Both RadarBox and BaseStation will display the callsign as they do for any other ADS-B aircraft, except that obviously no lat/lon will be shown.