Issue with change out-of-stock to in-stock

Hi, my store is a 60 products, each of them with 13 variations.
Stocks are very limited to 1/2 units by variation and due I sell same products on a real store, I need to daily manually modify stocks on woocommerce, using Woocommerce Stock Managaer.
My problem is when I’ve got a out-of-stock variation and then I add 1 unit of it, save and reload page, this variation is grown to 100 unit, causing problems with customers.
Can you help me?