Every time I read about this, I find a variety of opinions. I wish we could get a Kodak chemist to straighten us out, but that seems unlikely. Below is my understanding of the topic. Please comment, confirm, or disagree as you see fit.
The purpose of fixer is to remove the unexposed Silver Halide crystals from developed negatives. Improperly fixed negatives will have a cloudy look and will not be transparent. After fixing (assuming good fixer and proper times), your negatives should be clearly transparent, aside from a small amount of film base fog. The presence of a color tint does not necessarily indicate improper fixing.
Some films have various coatings such as antihalation layers, speed enhancing layers, and probably other proprietary layers that the manufacturers don't talk about. One or more of these layers is responsible for the purple, pink, or blue colors we see. Typically, fixer will remove these layers, but that is not the purpose of fixer. When fixer fails to remove the color tint, it often leads people to falsely believe their negatives are not properly fixed. It is possible that fixer becomes less effective at removing the color layers as it becomes exhausted, but this is not a reliable indicator of your fixer's effectiveness.
As long as you are sure you're using good fixer, and you follow the mfg recommendations, any remaining color tint should wash out with hypo-clear and a final rinse. If this doesn't work, try changing your washing methods before changing your fixing methods. If the film is already fixed, then fixing it again is really just using your fixer as a washing agent.
One thing that may help is a presoak. Try a 30-60 second presoak with water at the developer temp, and agitate. If you see a color tint when you drain the presoak water, you've already removed some of the color.
Paul