What does it look like?
Does it happen to dark colors mostly?
Do you use a lot of Odorless paint thinner/turpentine to dilute the paint?
Is your canvas cheap?
Is it canvas board?
Oil paints especially dark colors mute in the color as they dry. It is normal process. It is called "color sinks". Darks become some sort of chalky.
Canvas boards and very cheap low quality canvas drink all the oil from paint and it sinks to the back side of canvas. If you see oily spots on the back of your canvas, this is the reason. Painting will most probably die in 5-10 years as paint layer will disconnect from canvas. (Read the article of mine in section article how to select canvas, i had a lot of troubles with them till I found my manufacturer that I am happy with). Solution is to give 1-2 extra layers of gesso. But still see above about dark colors.
If you use a lot of thinner or turpentine it may happen everywhere. Also it promotes travel of oil to the other side of painting.
Solution is always the same to restore fully or to some extent:
1) Let it fully fry depending on thickness of oil paint film. 6 months is recommended.
2) Use artistic varnish to cover when it is dry. ( there was something about varnishes on the forum)