Datagrid's scroll bar thumb moves but data doesn't. (Solved)

[James Hale]

I seem to have solved the datagrid not scrolling issue.
Quite by accident I reduced the datagrid area in order to add a new one and try some different method of populating it. Belief in suspicious behaviour I know.
In any case before doing more than changing the original's size I looked at another issue and inadvertently scrolled over the original datagrid.
It scrolled!
I then resized the datagrid back to where it was and the scroll stopped working again.
I then realized that the top edge of the datagrid was under the bottom edge of another field.
Only by a pixel or two.
This overlap was the problem.
Or so I thought.
Playing around with how close I could get the two fields resulted in my not being able to reproduce the original problem.

So my issue is resolved, but I don't know by what means.

James