Can anyone please give me some insight on how the PM Request Generation works internally?

We have a PM plan loaded, that has been running quite happily for 6 months.  We release  tasks gradually, 4 weeks at a time.  We last released them up until August 15th, but now, trying to release up to Septemeber 12th yields no Requests/tasks.  Yes, there are tasks ready to be released for this time period, and all the tasks were originaly loaded at the same time and are attached to the same PM_Sequence.

What would cause them to suddenly not release? I have turned the logging levels up, but nothing really showing in there, no exceptions are recieved.  I can see a query being run on ‘contr_autogen_view’ that retrieves nothing.  There is also a query in the Log looking for errors (SELECT MESSAGE_ID, LOCALE_CODE, MESSAGE_TYPE, MESSAGE_TEXT from METRIX_MESSAGE_DEF_VIEW where MESSAGE_ID = 'InvalidXMLRequest' and MESSAGE_TYPE = 'ERROR' and LOCALE_CODE = 'EN-GB') but this does not return anything when i run it.

Any clues or suggestions?