I have 'Henry Bockstoce' and in my experience, the bud turns red, you think it will open, but it takes a whole. 'Red Grace' acts the same way here. You have this nice colored bud for over a week, thinking "any day now", only to feel like you have waited and eternity.
Then finally it opens up into that beautiful specimen you've been waiting for.
That said, this year has been miserable as far as weather conditions here for us in the Midwest. If you have now had this intense heat like we have, so early in the season after cold wet conditions and freezes during bud development, then this could also be playing a hand in your peony flower development.