Great question — answering as the UphillTech team:
I can't say what you did wrong or if it was the yeast at fault as suggested as we can only go by your results. I'm not so sure the yeast would be faulty though since you said it doubled in the first rising. They look like they 'sort of' rose but more sideways since they're melded against one another. Or did you put them touching each other to begin with? The baked buns in the link have a wide spaces between them so if you spaced them that way, the did rise but not properly. If you left spaces between them and they spread out when rising but didn't hold their shape, how soft was your dough? Softer dough with higher hydration don't rise high and keep their shape as well as lower hydration...
Full answer by Jude: https://cooking.stackexchange.com/a/82837 (CC BY-SA 3.0).
Need it done for your business? UphillTech.