Hi Srihari,
You need to check BOM status in CS02 / CS03 and not in production order. If the status of BOM header is set to "Inactive" in CS02 then system will not explode BOM while creating production order. User can still create order but system will show BOM status as 0 in production order CO02 / CO03.
In you case, I guess BOM was made inactive for some time. During that period user created 4 orders for which BOM was not exploded adn them it was again set to active status.
please check this possibility in your system,
Regards,
Rohit