bpc_send in beepcore C library hangs

When sending a reply from a listener implemented using beepcore-c
libraries bpc_send() is hanging. 
 
I ve tracked it down to the following call
 
fiostate_unblock_write() in bp_fiostate.c .
 
The listener is hanging when sending its reply at the following line
highlighted.
 
bp_fiostate_unblock_write(IOSTATE *ios){
 
    SEM_WAIT(&ios->pn->lock);
    ios->pn->sessionfds[ios->iostate_index].events |= POLLOUT;
    SEM_POST(&ios->pn->lock);
 
}
 
What could cause this to hang in this manner? 
 
Thanks,
Des Smith.