As you've discovered, package offers with Free Dining are not typically valid during holidays when the parks are already crowded with guests traveling during school breaks.
I've found that not only do I need to pay close attention to the dates that the
Special Offers include but also to the last day that the offer can be booked. In my experience, your best chance to get the discount at your first choice resort is to book as soon as possible once a discount is announced. The rooms available at these rates are often limited.
The package deal with free dining for travel dates May 27, 2011 - June 2, 2011 was released on August 16, 2010 and had to be booked by December 20, 2010.
Watch the Special Offers for any new discounts that may be released!