(1) If your radio or talk group has been disabled, you will not be able to log into the network.  (2) You are out of range of the network. (3) Your radio is locked in site lock which displays the flag ICON on the display and you are out of range of the site that is locked on the radio.  (4) Also, if you have swapped radios, you may not be able to log into the network because each radio has an electronic serial number (ESN) and if the network is checking the ESN, you will not be able to use the radio.