The (name of particular instrument) is too (loud/soft) on track X. Can this be fixed in mastering?

Typically not. Bear in mind that any instrument in a mix constitutes a wide range of frequencies. By trying to attenuate/boost these frequencies, other instruments will be affected.

The Corrosive Audio rule of thumb is the mix should sound good to you before mastering. If not you should redo it. Mastering is not intended for the fixing of mixes other than the overall tone and level.