When you check this in Manage | Database | Fields, is this a field of type calcualtion or a number field with an auto-entered calculation?
An auto-entered calculation field that references a field from a different record will not automatically update.
If it's a field of type calculation, check to see what table occurrences is specified in the "context" drop down at the top of the specify calculaiton dialog. It might be set to refer to the wrong "context". (Calculation fields do not evaluate from the context of the layout's table occurrence, they evaluate from the context of the table occurrence specified in this drop down.)
The field type for "Stock Value" is calculation.
The context is the Inventory table. I have two instances of this table: Inventory and Inventory Value List. I tried flipping the context to Inventory Value List and pulling the Units on Hand from the same instance. That did the trick!