The GL account that should be printed for the additional costs of an assembly is the 'Assembly Cost Credit' account, which is defined on the Control Account assigned to the Master Item. Check out the particular account set to see if this is set up.

Also, what version of Inventory is installed? If you are using the latest 8.0B, it may be this is a symptom of a problem we have found and fixed (but not yet released) in sending these cost transactions from assemblies to GL.