Act on the preparation answer, in the room
If half the group stopped after a few pages, spend ten minutes reading one passage together and discuss that instead — you will get a better seminar than the one you planned. If they read it and still will not start, the problem is not preparation and no amount of waiting will fix it.
Do not say what you think first
In a group this size the tutor's reading is the strongest anchor in the room, and a question framed with it comes back to you almost unchanged. Ask the proposition flat, collect it, and give your own view after the spread is on the wall.
Leave the distribution up while they argue
A student who will not defend an opinion as theirs will happily explain what the four people at the left-hand end might be thinking. Something on the wall to point at turns a personal risk into a shared object, and it is the cheapest way into the discussion for whoever needed one.
A unanimous result means you picked the wrong claim
If nearly everyone lands at four or five you have asked them to summarise the reading rather than judge it. Choose the claim the text genuinely leaves open, and if the room agrees anyway, sharpen it on the spot and ask again — it takes thirty seconds.
Give it a name, not an owner
'The strongest objection here says the sample was chosen to fit' keeps the argument alive without making anyone defend it. In a group that meets every week, attributing a position is what makes people stop taking them.
Stop asking once they are talking
These are an opening move. If the discussion is running, do not interrupt it to collect another set of answers — a seminar that has become a survey has lost the thing you were trying to protect.