sql join not showing some records - sql-server

Question

I'm trying to find all customers who have placed an order but there isn't a callback after their last order date.
My current query:
SELECT dbo.[Order].CustomerId, MAX(OrderDate) AS OrderDate, NextCallbackDate, UserName
FROM dbo.[Order]
LEFT OUTER JOIN (SELECT MAX(CallbackDate) NextCallbackDate, CustomerID
FROM AccountCallbacks
GROUP By CustomerId
) callbacks ON callbacks.CustomerID = dbo.[Order].CustomerID
LEFT OUTER JOIN dbo.aspnet_Users users ON dbo.[Order].UserID = users.UserId
WHERE (PaymentTypeID IN (2, 3, 4, 6, 8))
AND OrderDate >= NextCallbackDate
GROUP BY dbo.[Order].CustomerID, dbo.[Order].OrderDate,callbacks.NextCallbackDate, UserName
ORDER BY dbo.[Order].CustomerID
Tables:
AccountCallBacks:
[CallbackID] [int] IDENTITY(1,1) NOT NULL,
[UserID] [uniqueidentifier] NOT NULL,
[CustomerID] [int] NOT NULL,
[Created] [datetime] NOT NULL,
[CallbackDate] [date] NOT NULL,
[Enabled] [bit] NOT NULL,
[CallbackTimeID] [int] NULL,
[GaryCust] [bit] NULL,
[NotInterestedReasonID] [int] NULL
Order Table:
[OrderID] [int] IDENTITY(1,1) NOT NULL,
[CustomerID] [int] NULL,
[UserID] [uniqueidentifier] NULL,
[OrderDate] [datetime] NOT NULL,
[PaymentTypeID] [int] NULL,
[PaymentStatusID] [int] NOT NULL,
[PaymentDate] [datetime] NULL,
[TransactionRef] [varchar](50) NULL
And the aspnet_Users table is the usual .net membership users table
EDIT:
Apologies! I never actual said what was wrong! My query doesn't give me what I'm expecting one of the rows of data for a particular CustomerID isn't in the result set. And no, there should always be at least one AccountCallbacks.CallbackDate for every CustomerID as I'm joining on the dbo.[Order] table and they wouldn't be in there without first ever being in the AccountCallbacks table.
Feel free to ask any other info, help is greatly appreciated.