Hi Hasna,
You are probably using the subtransactions which are assigned to internal main/sub.
You must assign the subs which are not assigned to internal main/sub transaction. These are for summarization of the FICA document to determine receivable account, however in rate steps you have to use the subs that are for revenue account determination.
Assign the subtransactions which are there in transaction EK02.
thanks,
Dhiraj