Dual-band mobile antennas in general have some phasing network that
allows the antenna to have a good radiation pattern (gain at the
horizon) on both bands.

They also need to have impedance matching on both bands.

Unfortunately, there's nothing universal about dual-band designs as far
as continuity or shorts or anything like that. Additionally, a search
on "Comet SSB-7" brings up some fairly unfavorable eHam reviews, many
of which mention DOA antennas, having to get replacements twice, etc.

There's not much you can do without knowing all the specifics about the
design, even then it would be tough to fix whatever's wrong; something
you can do is to check are the coax continuity to the NMO mount you're
using.

Dan