DUPLO Train Action Bricks I have the weirdest thing with my son's set.  Currently one of the yellow action tiles honks the horn but the other yellow brick does not.  No matter the lighting conditions, the position, orientation, etc. They both used to work and suddenly one is specifically not reacting. If it was different colors not responding I would assume battery issue is affecting the color sensor, but they are both the same color surface finish.  Any thoughts?
You may want to contact the LEGO customer service about it and see if they can provide a replacement brick. Assuming your train is 10874 or 10875, they should still have it in stock and you can probably just order a new one via a replacement part order.

On the other hand, since the recognition appear to be indeed color-based this might not help at all.