If it is a dbox you can check the signal strength
The main cause is the signal. Either the feed isn't connected to the box or your signal is too low. Press REd then BLUE and note the SNR %. This needs to be above 55,000 before you will get any channels from a scan or 'time'.
Ideally it needs to be about 62-64,000 before you'll clear all channels. If it's inbetween these values, you risk missing channels from a scan or exisiting channels freezing or stuttering.
Also cheap cables with no screening can cause problems