It might be the material as well, or could be the geometry. You can select a face there and with Shift + G to select similar, go with Co-planar, is they are perfectly flat that option should select them all, if not then you have to do some good old scaling to zero in the axis. It could also be that you are using a bevel modifier and the indents of the circles are not deep enough. And it could also be the angle in your shade smooth settings.
You can also upload your file and paste the link for us to take a look if you want.
