Hi Wyndham,
Welcome to planDisney! I am delighted to hEAR that you are planning a trip to the Walt Disney World Resort and certainly want to assist you with this question!
First, make sure you have that 
vacation package or 
resort hotel linked in My Disney Experience. If it is not linked in your 
My Disney Experience account, that might explain why you cannot make all of your 
advanced dining reservations. To link it, create a My Disney Experience account, login, click on My Plans, scroll down, and then click on the plus sign to Add More Plans. From there, click on 
Link Reservations and Purchases. Use the confirmation number to link the reservation.
If you do have the vacation package or resort hotel linked, double-check the dates on your reservation. If the reservation is linked and the dates are correct, then I suggest you 
contact a Cast Member by phone. They can help you identify any problems with your My Disney Experience account.
Also know that 
Disney Resorts Collection Guests can make dining reservations for the length of stay up to 10 days, so if your stay is longer than 10 days, that could explain why you are not able to make all of your dining reservations. Time is of the essence, so I do suggest that you contact a Cast Member immediately if your resort hotel is linked in My Disney Experience, it has the correct corresponding arrival and departure dates yet you are still unable to make dining reservations.
Wyndham, keep your eye on the 
operational updates as well since 
dining experiences could change. I trust you will love your stay at Disney's Grand Floridian Resort & Spa. We stayed there on our last visit and enjoyed using the walking path to get to Magic Kingdom. In the meantime, find your 
superhero gear, gather your crew, and get ready for an 
Incredible Disney Day!
Magic and memories,
Donyell