Last night i almost gone crazy, because no redirection occured, when i pressed on specific button. Only thing i have done, was, settings the session for the first time. Since searching for an answer to solve this solution, angry like hell, i found out, i never learned, how Response.Redirect really works (this ). After scratching and studying on a white paper and reading ASP.NET forums ( i decited to put it all together), it is really simple mechanism. When response.redirect start's, add special header to the object and send request to the server. After that Response.End is called and current thread is aborted, page execution is ended.
my code:
Session["SomeKey"] = "something"; //add something to the code
Response.Redirect("~/testpage.aspx")
Well, here is the catch. When session is first time created, server creates a special cookie, where session token is created. On all further request, server looks into this cookie, and find appropiate session token ( if you are using cookieless session, then session is saved in a url - for more info, read this ). Now, he knows, which session to call. But if you do upper situation, it maybe wont work. Why? Because Response.Redirect stops execution of a page, the session token is lost in translation 
Best way to get things really working is to use
Response.Redirect("~/testpage.aspx", false);
or use another type of redirect ( like Server.Transfer ... ). Differences founded here.