Is it true that in the new multi scan weather radars fitted in the A320, low level cumulus clouds are not picked up?
I'm not sure where you got your info from, but here is what I know of the Multiscan radars from Collins and Honeywell:
Multiscan displays precipitation from the highest tilt to the lowest so if the cloud is dense enough, even if it's a low level cloud, it will still cause returns. Keep in mind - just because the clouds are cumulus doesn't mean it has enough density to cause the WX-Radar to display any returns.
If you're talking about the dark-cockpit-concept, then yes, at certain high altitudes, the clouds will not show on the radar. This is because the OEMs want us pilots to only see what will pose a potential risk of turbulence and safety while at that particular altitude. If you want to see what's directly underneath your aircraft, the radar should be set to manual mode. However, this only happens during cruising at high altitudes.