Hello,
Even though you are based in Vietnam, I guess human nature is the same everywhere.
Further to what B.Saikumar & Nathrao mentioned/suggested, are you sure they HAVE sufficient work to do to cover the full working day time?
Prima-facie, the employees surely DON'T 'enjoy' what they are doing....else 'getting bored' doesn't arise. 'Tired/fatigued' Yes, but not bored.
Rgds,
TS