I will ask the obvious question first. Have you connected the second set top box to the aerial? If so, have you tuned the set top box? If both answers are yes, is the aerial going to the set top box then on to the TV set?
Do you get a good analogue picture on the TV?
Is the set top box connected to the TV via scart leads? If so you may need to switch your TV input, using the remote control, from TV to AV.
If it still doesn't work try moving the TV into your kitchen and connecting it to the original set top box. If that fails it's difficult to suggest anything alse without seeing it.