Welcome to Beoworld !
There are a few things to check;
Make sure that you have connected each pair of speakers to a left/right pair of connectors respectively.
Speaker set 2 has a switch at the extreme lower front of the Beocenter (not all owners are aware of this).
Check also that the balance control slider is centered.
If all this is checked and the problem remains, it will have to be an electronic fault. Most likely a capacitor. There
are a couple of tiny green radial ones on the board near the cutout for the dial gear. I've seen them fail many times.
If the missing channel is related to the radio section only, with tape and phono working fine, the problem is usually
in one of the MPX filters. They can go o/c for no apparent reason.
Finally one of the amplifiers could be bad but that is a bit rare to see in this model.
Martin