create po in backend

hi

I am on SRM 7.01 and using the classic scenario

process

• SC created in SRM - > SC goes to approving ->Manager approved the Sc -> system create PR or PO or RS Automatically in backend as a follow on doc

problem: 

when purchaser create sc and manager approve it no follow on doc created until i run FM :BBP_PD_SC_TRANSFER

and when i run this FM PR and RS are created in backend but no PO are created

i have 2 question

1. why i need run BBP_PD_SC_TRANSFER to created follow on document?

2.why i cant created any PO?