Paper's preservation (fixative not working)
Paper will yellow over time because of lignin in the pulped wood.
Choosing an archival, acid-free fixative means that the fixative itself should not yellow over time. Unfortunately, the primary purpose of the fixative is to prevent smudging of the media that is on your paper. While it can slow down paper yellowing over time (by reducing the surface area of the paper that is exposed to air), it doesn't neutralize those oxidizing chemicals.
There are ways to treat the paper to prevent the yellowing from getting worse, and you can also put it behind UV-blocking glass to protect it from further oxidization due to air and light. However, the current yellowing may not be reversible.