The JV 1080 (and I think the XP-50 should be the same) has a GM RX(receive switch) and if it is set to off GM won´t work. This switch is located in the System Page. Also when I want to play a GM SMF I manually put the JV in GM Mode which is separate from Perform Mode.
About the bank selection, well it could be that tha bank change switch (also in system mode and in each performance memory) could be set to off. Also, remember that if you are playing multitimbral songs you should be in Performance mode.