WCF service is not working properly

Question

  i  am using  wcf service  for storing the  string (in file) at the particular  location in  my  machine,

if i  use this  service in  my separate  project, and i  used it, sometimes it do the  storage ( as the code defined in the service)  &  sometimes it  shows me the  previous content in the file...

i am using  file stream in  my application  , i am create the  file  using  file mode  - filemode.create


why this  problem  arise  ,  plz help  me..