IPhone -Pthread Status Signal Does Not Unblock Pending Thread

I have two threads that execute as follows. The code works fine on the iPhone simulator, but not on the device. On the iPhone (there is ios 5.1 running), thread A signals a status, but thread B waits indefinitely.

In stream A, when the data is ready for processing:

pthread_mutex_lock(&mutex);
outstandingSig++;
pthread_cond_signal(&condVar);
pthread_mutex_unlock(&mutex);

In stream B:

 while(1) 
{
  pthread_mutex_lock(&mutex);
  while(outstandingSig == 0)
   {
    pthread_cond_wait(&condVar, &mutex);
   }

  outstandingSig = 0;  //Reset outstanding signals
  pthread_mutex_unlock(&mutex);

 // process data
}

Any suggestions why it behaves differently on the device? What could cause stream B to not consume a signal? And why the different behavior on the simulator and device?