Just type "Billing Split" in Google and execute. You will find many times this has been discussed here. Take the inputs from there and check your data. You may also have a look at the following notes:-
G. Lakshmipathi
Dear Lakshmipathi,
My Scenrios is like below.
For the same sales order item 2 Deliveries are created, but for one delivery document invoice document is created through VF04 transaction.
Now I have created the invoice document for the missing delivery by individual not with VF04 and analyzed with split analysis and I find below reasons.
1) Billing date for billing index dates are difference for both invoice documents
2) Number of foreign trade dat i values are different.
Where can I check these values, could you help me above are the reasons for invoice split. please share your ideas on this.
Thanks & Regards
MK.
1) Billing date for billing index dates are difference for both invoice documents
2) Number of foreign trade dat i values are different.
Both are self explanatory. First one is billing date is different which you can check FKDAT. Second is at header level, you can see a tab "ForTrade/Customs". Go to this tab, so that you can see an icon 
Click this so that system will take you to complete the missing fields value. Fill those fields and save.
G. Lakshmipathi
Dear lakshmipathi,
The issue when we create invoice through option collective billing document /online from VF04 transaction out of 5 deliveiries invoice is creating only for 4 delivery documents, but when we try to create with option collective billing document (with out online) from VF04 , invoice is creating for total 5 delivery documents.
If you any idea could you please tell me why its behaving like this.
Thanks & Regards,
Mk.
