stock adjustments

Hi,
In the past few weeks I have noticed that our inventory is not being accurately updated in response to online sales. In some instances the stock will adjust correctly, but then the next day might show two additional items that are not in stock.
I also use the WC Stock Manager plugin, but I think the issue is at the product level.
It is very important for our business that our inventory is accurate at all times, so I would appreciate a solution.
Thanks

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