Perhaps this goes to something like in the Brisbane floods case. There all cases that involve an "economic loss" only , not a direct loss to their property are being excluded from the class action. This on the grounds that economic loss is too hard to prove. This doesn't mean that persons who have suffered an economic loss have no case, but rather that IMF-Bentham & Maurice Blackburn (in that case) want to avoid levering the 'hard' case off the claimants who have an "easier" case.
'Too hard to prove' would not be their terms, as that'd be too crass for lawyers, but in effect, that is what they have done.
If that is the logic, then perhaps those shareholders who took positions earlier than when UGL had a responsibility to report, had done so on an a fair evaluation of the Ichthy's JV and had hoped and evaluated a potential to profit (or sustain loses ) - so such longer interests would be similar to the "economic loss" indirectly occasioned by the non disclosure.
They may have their reason enough to act on their own?