Notice that all of the pairs are bad, and all of
the pairs have the pink electrode.

So, maybe your pink electrode is bad, and
the others are good.

Or, perhaps the green, red and blue are all

If your system lets you select which electrode
pairs you want to test, you could then test
green to red, green to blue and red to blue
to try to figure out which one is bad.  Many
systems don't let you do that, however.  In
that case, your best bet is to re-do the ground
(common) electrode; the pink one in this