Solids can be problematic and I rarely work with them myself. There are a couple of plugins to help find problems with them. Solid solver is one that springs to mind, but there are others. Just looking at your image the bottom surface looks blue, is it? I think if you have faces the wrong way around it won't create a solid. Change your view to monochrome and make sure it's all the right way round. And see if you can spot any holes or loose geometry. It could be something as simple as a tiny line.