Mark,
Is the inventory warehouse managed or inventory managed? Usually, if you are in an inventory managed scenario, the system lets you directly input a picked quantity directly on the delivery. If you are in a warehouse managed scenario, you have to create a transfer order to do picking. You can check this by looking at field "OverallWMStatus" on the "picking" tab of the delivery. If the status says "WM Transfer Order required" you are in a WM managed scenario and have to carry out picking via a transfer order.
More information here.
Otherwise, I am not sure what the issue is!
Krishna