'Staff only' toilets smell bad

WHAT causes the smell on the Star Ferry's World Star, (Tsim Sha Tsui-Central)? I suspect the toilets.

These are 'staff only', so one can only guess.

The smell seems worse in the afternoon, perhaps the toilets need to be cleaned more frequently.

Or is this assault on our nostrils designed to force us to take the upper deck? I am sure that many fellow passengers who take the Star Ferry would agree with me that relief from this smell is long overdue.