Charts created with a pattern fill (especially a "hatch" pattern) may make the value labels unreadable on screen and on printout. This could happen with any of the Chart types value labels including percentages in slice with a PIE Chart.
When printing with the Color/Grayscale print option turned on, the percentages will be shown in white. When printing with the Black & White print option turned on, the values will be displayed in black. In both cases the numbers will be difficult to read if an alternating white and black pattern fill is used.
Some Things You can Try:
1. Try changing the font size or style so that the label value is more readable. In order to change the font or style it is necessary to first select the Series box in the Legend corresponding to the slice, bar , etc. that has the value you wish to change. A white circle will appear in the center of the Series box indicating that it is selected. At this point you can choose any font, font size or style and the label value will change.
2. A less desirable choice would be to not use your preferred pattern. Use a solid fill or a pattern fill that does not alternate from white to black.