Issue with Session Manangement

I’ve one issue which is causing a problem at production. I’ve web service written in ASP.net and issue is related to the session management. When there are 2 or more session running simultaneously then application is not able to detect the subsequent requests from respective sessions and responding the data of one session to the other session’s request. This is happening randomly so there is no certain way to produce it. The way I have done the session coding is to create session when user login’s and store its respective data in session so that when that data is required it should respond it quickly and not fetching data from backend at that time. I cannot afford to avoid the session and send new request to back end all the time. Most of our applications are working on same architecture. When login function is called then first I empty the session variable and then after successful authentication I create new session and stores the data in it.