[Morphic flip]

If a compound with ⁹⁹Ru (or another for that matter) is the central ion, the
⁹⁹Ru spectrum is seen for one isomer, but not the other isomer, why is this?
I take it`s something to do with shielding and cis/trans isomers?

[Dan]

Two things,

1. Which spectroscopic technique are you talking about?
2. Give a specific example.

[Morphic flip]

Good point 
It`s NMR
The last part of the question is stating "A <sup>99</sup>Ru spectrum is seen for one isomer, but not the other, why/what is the reason.
I`ve got the isomers, obviously (to me) one is cis, one is trans.
It appears like a common (which I don`t know) answer why one isomer would show a spectrum and another not.
Like a general rule or something.

[Dan]

Isomers of what compound, it makes a difference?

This is probably due to very fast quadrupolar relaxation in the lower symmetry isomer, but without knowing what the compound is, I can't be sure.