(I regret that I am not able to offer Live Phone Call service at this time; However, your request is still open to other Experts in this category, and one of them may pick up on it at any time.)
Does the white line constitute interrupted toner, of all colors? try printing a full-color sheet, to see.
If so, the developer or 'drum' may have a damaged stripe upon it. That, the drum, is easily damaged. Such damage should show at a glance.
Note that 'drum' damage usually shows a slight repetition down the page at intervals of the circumference of the drum.
Another possible thing: toner spills upon the corona wire. Cleaning the corona wire, now usually included in toner cartridges, by moving back and forth the small colored-plastic tab on the toner cartridge, should eliminate the white line of missing toner, if that's what is causing it. (And that would be the case where toner is missing in one color but not others, most likely.)