All offers have an expiration date.
Mousesavers may list the booking date of the pin. I think they list that info on offers.
If the offer did not expire, that probably meant there was no availability for the room type/resort you wanted for your travel dates, for that offer.
If you call back, ask them if the offer expired, and when. Then you will know if the issue was an expired offer or room availability.
Although the CM did say it expired 06/03. So that is probably the case. But you can always double check.
If you are just trying to add days to an existing booking, as long as it is within the travel dates of the offer, I believe I have read here a supervisor can modify your res. And they have to if the offer has expired.