Why Does Poor Mass Transfer Increase HETP?

Points on how poor mass transfer increase HETP are:

• HETP increases when mass transfer between the mobile and stationary phases becomes inefficient.
• Reduced transfer efficiency requires a longer column length to complete each theoretical separation stage. 
• This directly raises the height equivalent assigned to each theoretical plate.

Also check it out details on definition of HETP in chromatography to learn more about it.