I think the printhead needs replacement

Question edited for clarity and spelling 'alignment'.

It appears to be a known issue on that model plotter.

https://www.google.com/search?q=plotter+hp+dj500+alignment+failure

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Replace printhead and platen roller at the same time

if it is the least setting for print head then you may need to dry ribbon by putting in sunshine for few hours

Yes it might be a problem with the print heads. You can try cleaning printheads under tools menu. After cleaning printheads click on Align Printer in Tools menu and that will solve your problem. If not then you might need to replace the printheads. If you these print heads are less than 2 years then HP will replace it for free.

Sounds like there are some elements worn off of the printhead causing some of the bars to not be printed. To test, create a label that prints a thick black line or solid black box across the width of the label (assuming a 4" label). Any vertical white lines that show indicate elements that are not functioning.
You could also put one label above the other, lining up the barcodes and you'll probably see bars that are missing from the label that won's scan.
The only ways to correct the problem are to install a new printhead or move the barcode field on the label so that the missing elements fall in the spaces between the bars.

Hope that helps !

Have ou gone into your menus. Into troubleshooting, then image quality.
You will want to run both the eliminate light stripes and eliminate smears procedures. that will purge the print head, blow air through the jets and hopefully resolve the problem. I do believe the 8550 has a Jet Substitutin procedure where you can do a workaround and make other jets make up for the light areas.
here are some directions, Will forward more

1. Press the [Up] or [Down] arrow button on the Control Panel to select [Troubleshooting] on the Display Screen, and then press the [OK] button.
2. Press the [Up] or [Down] arrow button to select [Print Quality Problems], and then press the [OK] button.
3. Press the [Up] or [Down] arrow button to select [Troubleshooting Print Quality Page], and then press the [OK] button.
4. Evaluate the 'Light Stripes Test' page for missing printhead jets, such as missing Cyan, Magenta, Yellow or Black color bars. The 'Light Stripes Test' page is the last page of the Troubleshooting Print Quality Page that printed.
   
   WARNING: To avoid injury, do not touch the sharp metal strip attached to the clear rubber wiper blade in the Maintenance Kit.
   
   
5. Use a clean lint-free cloth or a new paper coffee filter to gently clean the maintenance unit's clear rubber wiper blade, use only a wiping motion across the entire length of the wiper blade, and then empty the waste tray. A dirty maintenance kit wiper blade can cause white lines and weak or missing printhead jets.





6. Repeat step 3 to reprint the Troubleshooting Print Quality Pages. If there are no missing or partially missing color bars then the print quality problem has been resolved. If there are missing or partially missing color bars then the printhead cleaning procedure will need to be performed. Continue with the next section.

To clean the printhead using the basic eliminate light stripes procedure:

1. Press the [Up] or [Down] arrow button on the Control Panel to select [Troubleshooting] on the Display Screen, and then press the [OK] button.
2. Press the [Up] or [Down] arrow button to select [Print Quality Problems], and then press the [OK] button.
3. Press the [Up] or [Down] arrow button to select [Eliminate Light Stripes], and then press the [OK] button.
4. Press the [Up] or [Down] arrow button to select [Basic], and then press the [OK] button. The printer will perform a head cleaning, which will take about five minutes. The printer will then print a cleaning page and a light stripes test page.
5. Repeat steps 1 - 4 one more time. If there are no weak or missing jets then the print quality problem has been resolved. If there are still weak or missing jets after two basic light stripe cleanings, continue with step 6.
   
   NOTE: Do not try the Advanced Eliminate Light Stripes Cleaning mode without performing two basic cleanings first.
   
   To clean the printhead using the advanced eliminate light stripes procedure:
   
   
6. Press the [Up] or [Down] arrow button to select [Advanced], and then press the [OK] button.
7. Select the missing jet color and number. If necessary, check the light stripes test page to determine the color and jet number of the missing jet.
   
   NOTE: Only perform the advanced light stripe cleaning once. If weak or missing jets still persist, continue with step 8.



8. Print the Troubleshooting Print Quality Page. Evaluate the Light Stripes Test page for missing printhead jets, such as missing Cyan, Magenta, Yellow or Black color bars. If necessary, perform the basic eliminate light stripes procedure one more time. If the Advanced Eliminate Light Stripes procedure does not resolve the weak or missing jets, perform the Enable Jet Substitution Mode. See the Related Items below.

First of all, clean the printhead.

if the lines are from top to bottom, some dots in the print head are broken. Replace.

If the lines are from left to right, check if the ribbon is loaded correctly, if yes, proceed to clean the printhead, and every part that are in contact with ribbon.

This problem is not a heat problem, set the temperature to 0, in order to save printhead.

Try cleaning the printhead, then print the zebra test print. (see user manual) or just print a solid black box 4" wide by 1" long. If you have white vertical lines through it you may need a new printhead.

If print test is good try increasing the heat/darkness setting

You will need a new printhead, you only remove to see if that's the problem, wrong or missing cartridge error means the printhead has gone bad.

This is definitely the print head. You will need to purchase a new print head and install this. It is very easy to install all that you need to do is lift the printer lid and extract the ink cartridges. The printhead is the black part under the ink cartridges and on either side of it is a lock and release bar. push these down and out wards to loosen them and then lift the print head out. replace the print head and then you will need to input the print head ID in maintenance mode on the printer. Turn the printer off and hold down the menu/set button and power on. You will here it beep and all the lights flash on the panel. The punch in 68 2580 I think from memory i could be wrong but just look it up if you can get hold of a service many and then enter the new printhead code and 99 to exit. Done Hope this answers your question.