Hi Kaushik,
Case 1 :
BOM can be used in different usage functionalities i.e. Production, Costing, Sales, Maintenance, Universal, etc..
if B component has the BOM usage other than Production (1) also system will show the assembly link
but if you click on that link system will give error saying the usage combination doesn't exist
Please check and update the status
Case 2:
if those components are added with reference to ECN then please ensure the validity of that particular ECN Number
Regards,
Venkatesh..