Using a CMYK Laser printer, why do certain colours not transfer when pressing the A and B Paper together?

The colours printed on a CMYK printer need to consist of around 80% toner coverage to allow the adhesive on the B Paper to take to the Toner printed on the A Paper.

Colours that do not contain 80% Toner coverage will not transfer correctly, meaning that this colour will need to be changed to a deeper or more vibrant colour to work correctly.

A rough guide to know whether a specific colour will work or not is as follows:

Any CMYK colour within your graphics software will show an individual value for Black, Cyan, Magenta and Yellow, that make up this colour. These colour values should add up to 80 or above for that specific colour to transfer correctly. An example of this is below.

Colour A consists of:
Black – 10
Cyan – 10
Magenta – 10
Yellow – 10

Colour B consists of:
Black – 25
Cyan – 25
Magenta – 25
Yellow – 25

Colour A show the values of each individual colour to add up to 40 – this colour will not transfer.
Colour B show the values of each individual colour to add up to 100 – this colour will transfer as the value exceeds 80.