I apologize for the confusion. When a sitter accepts a request it triggers a text message to alert the parent. If the sitter changes her response to decline/unavailable, a new text alert is not triggered.
Sitters sometimes accept before checking their calendar, misread the date or accept when they intend to decline and change their response.
You can add more sitters to your request by selecting: Bookings > Requests > (select the 'pending' request) and then select 'Add More Sitters' from the bottom of the screen.