Users Not Recieving Alerts

We have just migrated to Sharepoint 2010 and since the migration, users can set up Alerts, and recieve the Notification of the set up, but they don't actually recieve any alerts.  If I log into sharepoint as the Domain Administrator account and set up an alert for a fuser then this works, but we need to have other users able to set up and recieve alerts.  I'm assuming there is a permissions issue somewhere but I've tried most combinations and still not working as it should.

Any help much appreciated.

Duncan