problem about the network card driver!

Hello, recently I wrote a  virtual network card driver of vxworks and
suffered some questions .

The driver works as follows:
     When receiving the data, the driver receives MAC data frames from
FPGA and deals with them . Then put some MAC frames into TCP/IP
protocol stack of vxorks .  And receive the datas by socket program in
application level.
     When sending the data,  useing socket program to send the datas.
And get the datas from endSend() function of the driver. Then send the
datas to FPGA.

   Now, I have done the driver and loaded it manually.  The
muxDevload(), musDevstart(),ipAttach(),ifMaskset(),ifAddrSet()
functions all can be executed correctly and  the ifShow()and muxShow()
command shows that everything is correct.  But when I writed a socket
program for testing the driver, the endSend() function didn't be called
at all. It looks common form the loading process,  but why the
endSend() function not been called?