Jolin,
There is few check points you can check and configure and see if it works
Option-1-Check you config using tcode OKO7.Select the settlement profile which you want distribution rule to be 1
at bottom there is a field Max.no.dist.rls maintain 1 over there
caution- this option will not allow you to automatically add an extra settlement rule, should the need arise. each time you have to add manually in t-code IW32
and/or
Option-2
Define Strategy sequences for automatic generation in t-code KSR3_ORC
Refer below link
http://wiki.sdn.sap.com/wiki/display/ERPFI/Internal+Order+Settlement
Best regards
Hrushikesh