We are using large scale roller culture to grow Vero cells. We have recently found bare patches in otherwise confluent rollers (something we don't see when we grow the cells at smaller scale).

As shown in the image attached, often these patches have clear boundary lines between confluent or nearly confluent cells, and nearly bare plastic. I'm hoping somebody can offer an explanation?

We've cultured Vero for years without seeing this problem. We've already tried changing FBS, medium, dilution, roller manufacturer, roller speed and cell line. We've tried trituration of cells during passage. We also had a complete clear out of all reagents and fumigated the BSC to rule out contamination, but the problem persists regardless of anything we've tried. Any assistance would be greatly appreciated.