Queries :: Identify Subsequent Records Where Original Record Meets A Criteria

I have a table [PickData] in a WMS (Warehouse Management System) database, that records details of each item picked. The key fields are;

[Movement] - a unique ID for a collection of items to be picked.
[ToAssignRef] - the order ID
[Product] - the product!
[Picked] - the date/time the item was scanned
[Pick Actioned] - the date/time the [ToAssignRef] was completed
[Reason] - A code to indicate why an item could not be picked - AKA F3'd

An operative would be allocated a movement, connected to the [ToAssign Ref], containing a number of products to pick. If an item can not be picked for some reason the operative presses F3 and selects a reason (no stock, damaged etc). These F3'd items (other stock allowing) will later be picked on a different [Movement]. I need a query to identify the subsequent [Movements] and the associated fields following the occurance of an F3'd [ToAssignRef] & [Product].

I have a query, but it runs very slowly (perhaps due to the DB size currently 780K records). Is this the right approach, is there a better (faster) way to do this?

Code:
SELECT PickData.ToAssignRef, PickData.Product, PickData.Picked
FROM PickData
GROUP BY PickData.ToAssignRef, PickData.Product, PickData.Picked
HAVING (((DCount("[Movement]","[PickData]","([ToAssignRef]='" & [ToAssignRef] & "') AND ([Product]='" & [Product] & "')"))<>0));