Asp.net Session Expiration

it seems that a session in asp.net sometimes expires before its time is
supposed to expire? so for example i developed a client side that relies on a

if Session("Var") > 0 then

else
end if

to verify that a user is logged in with proper credentials. now sometimes
the session expires before the twenty minute mark and kicks the user out of
the client side because the Session("Var") is no longer > 0. can someone tell
me why this is happening?