Aloft (Beta)
My nameplates do not seem to be sensitive to mouse input, what should I do?
This is controlled directly by Aloft's "Frame>Enable Mouse" option; make sure this option is enabled.

As well, significantly altering the "height" (vertical offset) at which nameplates display on the screen relative to the associated unit "character model", via the "Frame>Packing Height" and/or "Health Bar>Position>Y Offset" options, can move the nameplate health bar and associated nameplate graphics out of some sort of magic/invisible area that Blizzard treats as "mouse sensitive".

Find a safe, crowded place (e.g. an auction house), enable the relevant nameplates (i.e. friendly nameplates), and adjust the values of the "Frame>Packing Height" and/or "Health Bar>Position>Y Offset" carefully, a little bit at a time, until you find an effective compromise between mouse sensitivity, layout behavior, and the style of display you want.

It is assumed that "significant" changes to the horizontal display packing/offset values of nameplates will have a similar effect, but this has not been verified.

Additional really gory detail (about Acapela's experiments with mouse sensitivity, and about what seems to be going on under the covers in Aloft) is available in this Aloft bug-report thread.