Weird trouble here.
Somewhere in the process, the object can't render the displacement map anymore, even in the preview window shows the right bump amount. At start it works well but somehow it lost it recently... I check all connections, all values seems OK, the bump image shows in the right place and should make a huge bump effect (like in my image #2. But nothing, the surface remains flat as a sand desert. It's an animated object but no keyframe on this parameter, all frames show the same absence of bump. I check there is no object's double superposed. I copied the object, I try tho connect the image directly to the Displacement node socket: nothing. I tried a Noise texture with Color Ramp to enhance the contrast and then, a very small amount of bump appears.
The goal is to have a small amount of relief, but here I exagerate to enhance the problem.
The object contains several materials. I tried to recreate a new material with new nodes: nothing change. I put simple Glossy node with another image for the bump: same result. Correct effect in preview window, nothing in render.
Bump mapping works on other objects in the scene.
Capture #1 The material preview show the expected result with these settings: a huge bump. Remark: there's two image for bump, one (top) for the bumpy surface, another (bottom) for slim lines barely visible.
Ok I got a temporary solution. The bump was still there but for some reason, it was so light that it was barely visible.
After the Color Mix node and before the Material Output node, I add a Math Multiply node (value x 20) and then the bump shows up again.
But now rendering, I see baaad artifacts in the bump texture due to A) the Math multiplier node who force the grey levels and B) a slightly poor image histogram. Photoshop's histogram shows some pikes. Looking closer, there is some posterize-effect in the gradient and its ugly in the render. The texture image is 4000x4000 pixels, large enough I think. I tried PNG and JPG, tried to remake it with more histogram values, same result.
- Edit: With Normal mapping it seems to work better finally. Still some poor histogram artifacts but better. I blurred it a little and its fine.
But still at some point the bump map went to almost zero for an unknown reason.