I’m guessing you’re seeing that you have inventory on hand by noting the numbers in the “Qty on hand” column when you view your inventory items? If you click on that number you should be able to see a break-down by location (if you have multiple inventory locations), and by clicking again you should be able to see a record of all movements in and out of that location for that inventory item. If you’re not seeing any record of it moving out, then there’s presumably something wrong with your sales or delivery transaction. My first thought is that perhaps you haven’t selected the location from where to take the inventory item when it is sold, so although it appears on the sales invoice and delivery note, Manager doesn’t know where to pull it from. (I haven’t enabled delivery notes, so I’m not very familiar with their functioning.)
If none of the above applies, or you are sure you have done it all correctly, perhaps you could post screen shots of the transactions involved? That would be the Purchase Invoice, Goods Receipt, Sales Invoice, and Delivery Note.
Also, if you suspect any issue with the software, it might be helpful to know which version and platform you’re using.