Just a point of pedantry:

3db = power ratio of 2:1 (halving or doubling)
10db = power ratio of 10:1

db are sometimes used for voltage or current in which case
6db = 2:1
20db = 10:1

As for the fault it's just guesswork without seeing it. No signal may not mean no signal: it could be inadequate to produce a picture. You could try moving the tv/freeview combination that works to the aerial connection that doesn't.

Stu