c450 print quality

[hooman23]

as you can see pictures i attached when i print halftone test print with 255 full color there is no problem but with 64 you can see small box and lines . i have this problem with all colors except yellow.. i dont know if its the problem of my copier or it should be like this. when i print shape with high contrast the quality is really high but when i want to print with picture with color back ground for instance sky with clouds . the quality is not satisfing

[rrrohan]

Well you will always notice quality issues at 64.
How old is the transfer belt?

[hooman23]

i think 4.5 k

[mjunkaged]

I don't see a box and lines in the samples, but I do see voiding / faded areas. Silly questions: Do the defects move around on the page, or same exact spot?
Have you tried printing on the backside of samples that are already printed /undamaged paper to make sure it's not damp spots on the paper? Have you cleaned the LED heads?
Is the customer printing HEAVY color all the time, like Photos full-page? They may be causing the defect by sucking way too much toner of those 3 colors on a regular basis.
SM 4-70 says the above (clean LED PH with wand) and image units, too dense of copies, etc.