What do you have connected up to the various speaker terminals?
You should check the speaker wire connections on both the amplifier and on the speakers, to make sure you do not have a loose wire somewhere.
What you describe sounds like you have a bad connection with your right speaker on the "A" connection. If nothing is properly connected to it, since that amplifier connects the A & B in series, it would prevent the B speaker on the right side from working when you select both, if you do not have the speaker properly connected on the A terminal. Again, check the connection at the amplifier and at the speaker.
If that does not fix your problem, post more in this thread and tell us what you have done, and what is connected to each terminal.