Hi Sachin,
Kindly check your master data of routing or recipe to identify work center.
compare both plant work centers and find the activity types which are assigned in that.
recheck all activity types in kp26 for current month.
Pl elaborate your issue while settlement(KO88) of process order so that any body can give you better solution.
Costing variant might be not a problem!
Regards,
Ravan