I've had immature plants do that before; the buds just seem to atrophy at the stage of development that you describe, and they never open. The plants seem to grow out of this behavior in subsequent years; growing them is certainly a long term investment!

Thanks for the answer bubba62. The Rockii is 60cm tall, is it still consider immature? Could it be due to the transplantation? or lack of sunlight? Now it gets only 4.5 hours direct sunlight daily from 9am till 1:30pm.

A mature tree peony could be anywhere from 10 to 100 years old. What you discribe just happens even with older plants.

Is your rockii tp a species rockii or a hybrid rockii? My hybrid rockiis seem to bloom about same time as other tps.

Thanks for the answers. This is the disadvantage of buying cheap tp. :) I bought it in a home improvement store for 10 euro (offer price) with a tag of peony rockii. I believe the tag was right, as I can see the typical rockii growth, but don't know what type of it. Probably it's wild rockii. So I just have to wait for few more years for the flower.:(