What is a hollow word ? What's the reason and solution of the hollow word?

What is a hollow word ? 

What's the reason and solution of the hollow word?

(The hollow word: The character or image are lack strokes.)

The reason:
print quality are too light or too hard;
The paper was used the opposite side;
Toner itself problem , such as the toner melting point is too high or the particles is too big;
The magnetic roller coating is too thin or too old.

Solution:
Replace the printing medium; 
Replace the toner cartridge.