Hi Sandeep,
Check Field "Req Transfer" in "Requirement Class" setting in T code OVZG. It should be checked.
AND
Check field "Req./Assembly" in Schedule line setting in T code VOV6.This should also be checked.
if the above 2 are fine, then check assignment of Req type to Item Category in OVZI.
Do let me know if this help you.
Thanks
Roshan