Have them give you the exact search they are running, and the results.
First, make sure that the search they are kicking off is a consistent search. Make sure it has a fixed earliest and latest value, and so on. If they are kicking off "last 30 minutes" on one head then later on another had, then of course the answer will be different.
Second, run that query yourself on each head. See if your results are the same or different from his. Ideally, limit the search to as small a time range and amount of detail as possible, as long as he gets a different result on each head.
If there are lookups in the search, or joins, then run those subsearches independently and check whether they are consistent on the two heads. Perhaps a lookup isn't propagating fully, or whatever.
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