Hi Prasad,
Assuming that you have already checked the above mentioned points, here are additional suggestions:
1. Please check if you have periodic transfer activated for SNP created orders, then your Preq might be available in /n/sapapo/c5. If available there, select the record and trigger the change pointers to R/3.
2. Check the application log in APO and in R/3 for errors, if any.
Regards,
Umesh