Usually the landlord/owner of condo unit will pay the maintenance fee.

Is there a reason why the maintenance fee is not paid? Like some of the owners claim that they don't maintain the place then they don't wanna pay the maintenance fee or something like that.

I think if you don't pay then they'll just reject whatever you request from them until you pay off the amount owed. If the amount grows to be very huge then they may take legal actions.