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It seems that only occasionally when clicking on Compact (in Data File Maintenance/Settings) does a compact actually happen. When it does, it usually takes a few minutes, and the PST file size is reduced significantly.
However, most of the time when I first come into Data File Maintenance ... and click compact, it just goes through the motions, and nothing really happens ... taking only a second or two, and the PST file size does not change at all.
What am I missing here ?