What happens when you click on the storage tab?
Grant
In addition to Grant's suggestion, is this a Clone blueprint? If so, the default storage size will always be fixed to the size of the disk on the template VM. You can add drives under the Storage tab.
I have the same problem, "Capacity" field of the default volume is greyed out and users are unable to change its value during request. It's basic VM creation blueprint with only one disk configured (required) and checked checkbox "Allow user to see and change storage reservation policies" (I think that checkbox doesn't work at all in my case). Users can add new volumes, but they cannot change this one with ID=0. There are screenshots of request and blueprint in the attachment.