asp.net page wait others server side / asynchrone page

asp.net page wait others server side / asynchrone page

I created an asp.net page for waiting ajax. I have one page creating
something that takes 30 seconds. On every step I change a session value.
I have another page for ajax, returning the session value for showing the
percentage of creation. But, I dont know why, my ajax page awaits the end
of the creation of my first page. So I only get the 100% at the end.
Maybe it's because I use VS development server and not IIS server. If this
is the problem, can I change settings of the development server for
asynchrone execution?
Or is it something else?