Why am I losing oven spring.

Hi All,

I have been working on perfecting a crispy hard roll with a nice open crumb. I have a recipe that is working well, with the exception of losing volume during the bake. 

Here is what I'm trying to find a solution to. I put the rolls in the oven and steam them for about 7 min. When I open the oven to release the steam they have great spring and are round like baseballs. The problem is that by the time I finish the bake they flatten out quite a bit.

Any thoughts or advice would be greatly appreciated!