Don't give up! Cancellations can happen, so keep checking for My Disney's Girl Perfectly Princess Tea Party by calling 407-WDW-DINE. The more flexible you are with your date availability, you will increase your likelihood of being able to grab a future opening. For example, when you call you can ask the cast member to check each specific day of your vacation, not just one day -- if that would work for you.
One thing that helps with getting reservations for popular events is that you want to make sure and call 180 days out, preferably early in the morning. If you are unsure about the date, do a quick Google search for
WDW Priority Seating Calculator; it will link you to a calculator to help determine the date. I sometimes even call 407/WDW-DINE to confirm the day I can call to make my reservations. If you are a WDW resort guest, you can actually make reservations for the first 10 days of your trip. Moms Panelist Erin has explained this nicely
here.
Wishing you the best of luck in getting your reservation!