It seems they were searching in the early hours of the morning - not a great time for most humans - but people seem to think that having an infrared camera is a guarantee of finding someone.
The fact that the two women were found clinging to a lobster pot seems to indicate they had no flotation device or survival equipment so if it was just 2 heads in the water in the dark that would be easy to miss, especially if the sea state was anything but calm.
The turrets with the cameras in will doubtless have an autoscan function which, coupled with the correct height and speed settings (we used to use 60 Kts and 500' for a person in the water) would give the operator 3 chances to spot the person as they passed from top to bottom of the screen.
Sounds easy? trouble is that you need good thermal contrast between the object and the background and with just the head visible (and being washed over by waves) that can be very difficult to detect..
Local media report they were on 2 paddle boards tied together so they were out of the water and not just 2 heads bobbing