This could be due to deterioration of the heads, which could be
1) permanent with age.
2) an indication that they need a clean or maintenance.
You could do a test print, which should give 6 colour bars and it should show that the nozzles are deflecting which may cause the banding.
There are quite a few nozzles with this machine so one could be missing or blocked and won't fire.
They could become missing through head strikes (where the head has hit the edge of the media), or not cleaning it enough so could become blocked or won't fire.
If you want to reply with a post showing an image of the head, I could ask Steve from
BP LightBrigade to take a look and give a quick diagnosis.