Have the users not getting it logged in on any other Mac and have the policy run to add the dock items for them? If so, this is a known issue when using the Once Per User setting. See here: https://jamfnation.jamfsoftware.com/featureRequest.html?id=45
Vote it up while you're reading if you want to see the behavior changed.
If that isn't the case, then something else is going on. Each user logging in for the first time on a Mac should get the policy run on it. But ONLY that one time. If they log in again somewhere else, the issue detailed above in that feature request comes into play.
