From personal experience with marine radar, the images generated with a range scale set to three miles or less are pretty useless due to sea clutter. Also at these ranges there is the possibilty of very strong radar returns activating protective devices that are designed to prevent radar transmitter energy from destroying the very sensitive receiver section. In effect, the more powerful the radar return, the more likely that the receiver will be desensitised and effectively nothing will be displayed on the screen.
This is a very simplistic explanation of why they maybe saw nothing on the radar screen, as there are numerous settings that all interact and affect the radar picture. Depending on the wavelength of the radar, cloud and rain reflections can also absorb or reflect sufficient energy to blind the radar receiver at close ranges.
I had several years mucking about with the marine stuff including installations and commissioning, if anyone feels the need to argue or debate the details via PM.