Cache.get 'reponses' return nil help! help!

Hi all,

I want to cache some objects (models) in memcache, it stores the data
successfully using (Cache.put statement) but while retriving the stored
data from memory using (Cache.get statement) it return nil value. It
goes something like this..

if Cache.get 'responses'!=nil
       @responses=Cache.get 'responses'
     @responses_pages=Cache.get 'response_pages'
     @advertiser=Cache.get 'advertiser'
    else

      if
@@security.checkSecurity(session[:userName],session[:securityToken],8)==0
        @advertiser = Advertiser.find_by_user_id(session[:user_id])
       @response_pages, @responses = paginate :responses, :per_page =>
session[:record] ,:conditions=>["advertiser_id=?",@advertiser.id]
,:order=>'id desc'
     Cache.put 'responses', @responses
     Cache.put 'response_pages', @responses_pages
     Cache.put 'advertiser', @advertiser

Any suggestion will be appreciated!
Thanks in advance