User name is changing

Hi All,
         User1 logs in with "User1" as UserName with his password. After it, he fills some form and clicks on SAVE button. After it record saves successfully, but in LoginName control shows some another user name who already logs in, on another machine. [I have number of users]
I checked the web.config file.. The access permissions are defined properly... then what is the reason of it? Everytime User name is changing randonly. So that I am unable to genrate reports.
anybody know this?
 
Thanks
...