Didn't you just say the banner you printed came out GREAT?
The reason you were asked to post pics, is because there are different types of banding, and the more experienced users here can be more specific about your problem when they SEE the banding you are having.
Assuming you have run a nozzle test print to verify that the print head is working properly here are some things to consider:
Adjust the Media Feed Rate on the printer.
1. If there are white gaps between the passes, lower the rate.
2. If there are sharp dark lines between the passes, increase the rate.
3. If the roll has a lot of media on it, the printer will feed at a different rate than with a roll that has very little media on it.
Check to make sure the inside edge of the media roll (cardboard tube) is not bend or frayed. If it is, this may cause a different feed rate from one side of the printer to the other. Try to get a perfect circle tube and use a utility knife to cut away excess part of the cardboard tube.
Temperature and humidity can affect the quality of the prints. For most printers you will want to keep the environment at 72°F ±7°F and 50% ±10% humidity. The further the environment is from these optimum settings, the greater the chances of seeing banding in the prints.
Maybe your printer is colder on the left side?
During summer times, high humidity in the air will cause the ink to not dry fast enough.
1. Raise the heat settings
2. Print slower by changing the print speed in the ripping software.
3. Print uni-directionally instead of bi-directionally.
4. Increase the number of passes.
5. Decrease the humidity in the air.
6. Print at the highest number of passes for your printer
During winter times, low humidity in the air will cause the ink to dry too quickly
1. Lower the heat settings
2. Print faster by changing the print speed in the ripping software.
3. Print bi-directionally instead of uni-directionally.
4. Lower the number of passes.
5. Increase the humidity in the air.
6. Print at the highest number of passes for your printer
Because of the low humidity during winter times, static electricity may cause irregular prints. Anti-static mats and grounding the printer may help with banding issues. However, the only sure way to solve this issue is by raising the humidity around the printer itself.