Hi Bill, I believe that the problem is that your cut depth is too deep for your project, at least with the horizontal operation. What happens if you try a different operation, like a parallel path with the same step size? This would narrow down the problem to the horizontal operation itself, and (almost entirely) eliminate the possibility of the problem being a systemic issue - localizing it to the operation itself. There's the possibility that even if your project has a 0.1mm dimension on the Z axis that the image itself isn't exactly 0,0,0 RGB black on the background, which would cause the 'bottom' of the project to not fully reach 0.1mm deep, and instead be something less - which is a problem that we'd know to be the situation here if you were to try a smaller cut depth and it caused a toolpath to be produced (e.g. try a 0.05 cut depth with the same exact project setup).
The horizontal pass is a bit of a work-in-progress, and can be rather temperamental at the moment, ultimately requiring a complete re-work due to the fact that it has issues properly organizing the horizontal plane contours it calculates to piece together a toolpath. Aside from that, even when it does work properly it will only produce cuts at 'cut depth' intervals up to 'max depth' - provided that the input image has dark enough areas to reach at least 'cut depth' for the current project's Z size. So, if no part of the project is even 'cut depth' deep then it won't produce any toolpath. The project must always be exactly as deep, or deeper, than the cut depth itself - for the horizontal operation. If your project is 1.5mm deep and your cut depth is 1.0mm then you can expect the horizontal operation to only generate a toolpath that removes material at the -1.0mm Z depth, but no deeper. In other words: it won't automatically detect the next deepest level after -1.0mm, because there's not necessarily such a thing being that the input image is really just a varied surface/topography: it doesn't know that there's a 'top' and 'bottom', it just see a heightfield which it generates a toolpath to incrementally remove material from at successive cut depths that do not go deeper than the input image or max depth value.
So, a 50% solid gray input image that's used to create a project with a 1.0mm Z depth wouldn't produce any toolpaths with a horizontal operation that has a 1.0 cut depth - because no part of the solid gray image at that project depth would yield any areas deep enough to remove material from. The 50% gray would mean that the deepest part of the project is only 0.5mm deep, and a horizontal operation with a 1.0mm cut depth will produce toolpaths only in areas that are at least 1.0mm deep.
Another example, just to paint a clearer picture of the horizontal operation's behavior, would be the toolpath generated from a solid black image for a project that is 2.5mm along the Z dimension. With a horizontal operation that has a 1.0mm cut depth this would only produce two Z layers of concentric toolpath contours, one at -1.0mm and another at -2.0mm, leaving 0.5mm of material leftover due to the fact that the horizontal operation only creates toolpath contour layers at 'cut depth' increments along the Z axis, and does not exceed the 'max depth' parameter or the project's Z depth.
If you can determine that it's not an excessive cut depth value then we'll be able to investigate further, but right now I'm pretty confident this is just a cut depth error - one which I should add some kind of notification when it's the reason no toolpath is being produced.