Changing http traffic by using LSP problem?

Question

Hello! I try change and add some data to http traffic by using LSP. To do that i use LSP sample from Windows PlatformSDK and change lpBuffers variable in function WSPRecv in spi.cpp. When i only replace data in lpBuffers than all work fine. But when i try add data to lpBuffers than lpBuffers is change, but real traffic not changing. Anybody can help with this?