Sago palms will naturally get yellow veining as the plants mature and grow. If it is a transplant this is also a sign of some shock on some plants, has it been recently moved?
Also sago and most other palms will show yellowing as a vitamin or micronutrient deficiency. Without a visual it is hard to tell you exactly. I have about 20 different palms and all at some time show some yellowing. Each is unique thing and can be an easy fix. I have only once lost a palm to yellowing or disease in 20 years. If you have a coop extention by you this can be an easy way to start diagnosing the problem.
Lastly is there any new growth? Is it affected? Have you fertilized? And if so with what?
Remember that palms, especially Sago and Areca need a long term slow release rather than a shocking quick fertilizer!