fprintf to a network file

on windows 2008 server,when I use fprintf to write a LAN network share file frequently,sometimes the file pointer jump back:

fp = fopen(filename,"a+");

...

while()
{

...
pos1 = ftell(fp); <====== pos1 == 1000
fprintf(fp,"%s\n",str);
pos2 = ftell(fp); <====== pos2 == 800
...

fflush(fp);
}


At the same time, the file contents will be disordered.
The latter record will overrite the previous record.



please help to explain it, thanks.