You've mentioned that you are using WebServices, which is a technique for binding the Grid on client-side. If that is the case, the grid does submit any data to be accessible on the server. Thus the data key values you are looking for are not available.
Here is another article that can be useful when you want to access cells, values, controls and more: Accessing Cells and Rows.
To get the DataKeyValue on client-side, only the ClientDataKeyNames have to be populated.
Once using client-side binding, the following server binding expressions will no longer be compatible:
Instead, you would need to use the approach described in the: Client Templates
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