Typically occurs when the wrong amount of stock has been booked in.
e.g. You book in 120 on an ASN when in fact only 60 were delivered.
This leads to orders being released from back order ready for picking yet the stock doesn't exist.
Because the stock is allocated against orders you can't simply just book it out as it's need to be unallocated first.
1 - Check your Allocations settings - This is to stop the stock being allocated to other back orders
For 3PL Users.
Go Clients -> Find the specific client and click Edit.
For Multi Channel Users
Settings -> Client Settings
Then go the Defaults Tab:
Make sure the following is set to Manual.
2 - Unallocate the stock from back orders.
Two ways to do this depends on volume of products and orders involved.
2.1 - Order Overview - Bulk Actions
Selected the orders affected and select the "Unallocate Stock" option from order overview
2.2 - Product -> Manage Inventory -> Allocations Tab
Click the Unallocate Button from the revelant orders
3 - Once the stock has been unallocated then correct it by booking the stock out to get to the correct
4- Once that is done you might need to reprocess any affected orders if you have unallocated all of the stock and want to reallocate to a select number of orders
5 - Change the Defaults back to your normal preferences.