I am still trying to figure this out and have come across something very strange-
This is occurring in multiple work orders- all the ones I have checked have been created from a sales order.  When I scroll throught the quantity field in the work order, which is and has from the start been 1, a window pops up asking if I want to use multiple dates.  Defaults to no, and if I say yes it doesn't let me do it because the ones I have looked at already have costs issued to them.
I think this must be why the dates don't change.
Is there some flag that has been incorrectly set?  Can this be fixed?  It apears to be work orders created after a certain, as yet undetermined, date-- some work and some don't.
If this is happening when we create from the sales order we need to know how and why soon-  we create most of our work orders that way!  Is anyone else experiencing this?