Anyone having problem modify incoming IP packets at NetFilter Hoo k Module PREROUTING?

Hi All,

I am new to Linux and NetFilter.  I am trying to write a NetFilter Hook
Module that hooks to both POSTROUTING and PREROUTING filter.
All I am trying to do is, if an IP packet is for certain dest IP address, I
will add (POSTROUTING) and take-out (PREROUTING)certain number of
bytes in the packet before I pass on the packet.  The POSTROUTING NetFilter
hook seems working fine.  I can get all of the outgoing 
IP packets, and I can call skb_copy_expand + skb_push for the new data.