Producer / Consumer with Bounded Buffer, multiple producers/consumers with Semaphores

Question

I'm a bit stuck on the multiple consumer/producer problem. It appears in my lecture notes, but I simply cannot understand why the single consumer/producer approach won't work.

The typical approach for 1 consumer and 1 producer looks like this :

Producer : 
    while(true)
        emptyBuffers.P();
        mutex.P();
        buffer.insert(produced item);
        mutex.V();
        fullBuffers.V();

Consumer : 
    while(true)
        fullBuffers.P();
        mutex.P();
        buffer.consume(consumed item);
        mutex.V();
        emptyBuffers.V();

Why will this not work if I have more than 1 producer and/or more than 1 consumer? I've looked everywhere, but I can't find an answer I understand :s.

The mutex semaphore makes sure there are no 2 processes working in the buffer at the same time, so I don't see how this property can possibly change if you have more processes...

The 'solution' is that you change the mutex into a ProducerMutex and a ConsumerMutex. But now that would mean a producer and a consumer CAN be in the buffer at the same time, which should not be allowed, right?

I'm really not getting this :s