bose: front speaker off when rear?

hi,
I've been in it and everybody went out and talked.
I had already changed the front plates, the system went perfectly.
Then a hum started. had then separated the rear boxes and the buzzing was gone and the front voice worked.
Now I have replaced the rear platinums.
The strange one:

the rear boxes go, the front ones are off.
I'm stuck in the back, I'm going back to the front.

What can that be?

thomas