Perhaps they have been in storage?
Ivory needs to be used often and exposed to daylight so it can bleach naturally and keep its creaminess. So the best thing is to get the knives out (so to speak!) and use them daily.
Wash by hand in warm soapy water and dry immediately. Never soak or use hot water as ivory is porous and can split with hot temperatures, and water can seep into the joints and soften the glue attaching the handles.