how to avoid lose session?thank u very much

hi
   i 'd like to ask for help.
  sometimes,when the page is reloaded,or sessions are transfered between webpages,the sessions are lost(they become null)
  my web config is like that:
   <sessionState cookieless="true"       regenerateExpiredSessionId="true"                   timeout="50"></sessionState>
  is there any friends could do me a favor to give me suggestions?
 thank u very much in advance your kindness help
best regards
martin