After setting the number of test messages to a really high number things were finally looking up - the second and third and forth consumers were involved. But that sucked big time and to prove it I started digging further to find out what actually makes it do the things it does.
:prefetch => 1was the setting (as stated on some stackoverflow.com page) but setting it alone didn't do the trick because that made the channel get only one message and that was it. To continue receiving messages they have had to be ack'ed.
Talking is cheap - here's the example code