Even though the producers aim for a broad mix of views in the audience, it doesn't mean that audience members will be equally as demonstrative.
Maybe Labour supporters are more gobby than UKIP voters?
I think you scuppered your own suggestion of fixing in your question: there is no benefit to the programme if Dimbleby can't provoke an exchange of views either in the panel or the audience
Evidence: he did eventually get voices of UKIP support - so they WERE there
But he had to ask 3 times for them to speak up