As mouse said, 28 is thinner, 26 is thicker. So 26 should be making more blood come out than a 28 does. Either way, those should both be plenty big. It's the 33's you really want to avoid, and even the 30's can be too thin for some cats.
My hunch is that Gracie is a little dehydrated if she isn't bleeding well with the 26's. Make sure to add some extra water to her food, keep fresh water out for her to drink, and limit the dehydrated treats. And warm her hear and massage it a bit before you test to get the blood flowing up there.
Sorry her ear is being difficult for you!