oh yeah, that's not what I imagined. I was thinking developer streaks which can be an issue depending on you comb. of paper and time in the developer. Okay, so if you're following the correct chemical and wash procedures , the chemistry is fresh (does the staff replenish often enough); then it seems reasonable to say it's contamination. Another high possibility is sludge build-up in the developer tray (looks brown with a silver sheen) staining the back of the prints. Avoidable with proper scrubbing.
It could be your fellow printers and their handling of the tongs. Also, bamboo tongs absorb the chemistry and if the staff at the end of the day is not washing them thoroughly, or at all before the next day that could be one source.
Proper fixing and agitating the print for the first thirty seconds of fixing is important .A two bath fixing method helps ensure proper fixing especially with large volume printing.
You may find some non-abrasive way of confronting the other printers if they are being sloppy, or talk to management and see if they are willing to help.
good luck, cameron