can someone plase help!!!

when i first had this phone email notifications used to work ie i used to get email via gmail as soon as they were sent but after 2 days this is no longer working. I have phoned vodafone up and they say what the nexus one web site says. I might get email notifications when i am using the phone but when the phone locks i will not recieve any notification from any email providers gmail or yahoo. my old blackerberry gave notifications every time. as this is a google phone and software i thought i would not have a problem but it seems other people get the same problem.

please help