Nuclues Net Buffer

I have an wireless network driver for Nuclues. I need to know the
usage of MEM_Buffer_Dequeue & MEM_Buffer_Enqueue usage. The driver
works fine for few 40 iteration(ping). Then i get an Memory access
violation in the recieve function

When i say transmit i dequeue an net buffer from free list.[ pNetBuf=
MEM_Buffer_Dequeue(&MEM_Buffer_Freelist)]
 After transmit is done in my transmit done call back i call
[ MEM_Buffer_Enqueue( &MEM_Buffer_Freelist, pNetBuf) ]

in the recieve i call MEM_Buffer_Enqueue (&MEM_Buffer_List, pNetBuf)
pNetBuf is the netbuffer recieved. & notify the OS  using
NU_Set_Events(&Buffers_Available, (UNSIGNED)2,NU_OR) after recieve is
DONE!!!!

I have doubt in the flow. Is this a correct method of handling the
buffer and freelist. Is there any better way to do the transmit &
recieve.

Any suggestion is appreciated.
Thanks in advance