explain colors on Apple AA battery charger. mine flashing yellow

Here is a pdf of the Apple manual for the model that holds two batteries, found with a quick search of the Apple support page:

https://manuals.info.apple.com/MANUALS/1000/MA1545/en_US/Test.pdf

I cannot find one for the model that holds six batteries but I suspect the indicator lights work the same.

The most likely cause is they are not seated properly.

I should add, that assumes you are not trying to charge non-rechargeable batteries.