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Bad reception on just one Multiplex as a result of an insufficient aerial installation is quite feasible.
The difference in signal level between "just good enough to be perfect" and "not good enough to be perfect" and "insufficient for anything at all" is tiny. Different behaviour of different boxes with marginal signal can easily be accounted for by miniscule differences in tuner sensitivity.
The setup utility will show you the signal attributes for just one of the six multiplexes at any one time. If you happen to be looking at one of the "good" ones, then you might expect it to show good values.