From "Your Guide to Weather Radar" - published by Rockwell Collins. Page 19. Quote:
The reflectivity factor for some forms of precipitation is difficult to determine. Hail, for instance, occurs in many forms such as dry particles, water coated particles and melted particles. Additionally, hailstones are irregular in shape and do not provide a consistent return. Water reflects about five times more radar energy than solid ice particles of the same mass. Unquote.
Further to my above. At high altitudes where the tops of Cb may be invisible on radar due dry ice crystals, some radar manuals recommend you switch from Cal. gain to Max gain. This extra power will often reveal a small return from the tops of a Cb - enough to alert a cunning pilot at night or in IMC that something big ahead is waiting to belt the daylights out of the aircraft. Simply adjust the tilt for confirmation. I have trundled around a hundred such monsters using that technique.
Also if obvious unhealthy clouds simply refuse to show up on radar except at very short range, then tell the technicians to closely examine the radome for pin hole leaks. Water gets into the inside of the radome and freezes. This coats the inside with ice and the radar is severely attenuated - that is, has little penetration. Hence apparent short range.
It is usually ground tested serviceable because by then the water has melted. Then on the next climb the moisture inside the honeycomb re-freezes - and so on. We found this by photographing various Cb when spotting them visually and then photographing the radar screen. Then we sent the results to the Bendix Radar people in USA and they in turn told us to check out the radome. Our techs did just that and located the pin hole leaks. The radome is removed and baked in a special oven to dry out the moisture and the holes repaired. Hey presto! a fully serviceable radar next time.