Keep in mind that as you first start out, you are getting cold and hot mixing; then, as you try to make it hotter, you start decreasing the amount of cold. At the warm temp, you are getting the full flow of both hot and cold supplies, but as you approach all hot, you only have the hot line. This could be a supply thing, as the hot line, especially if it has any galvanized steel pipe in it, if probably rusting, which can close off part of the flow. It may never happen, but if it does, often starts on the hot side. Do you ever get any rusty water out of the valve, especially at the start (it often clears out after a little flow to purge it)? If the valve has in-line shutoffs, the hot shutoff might not be fully opened. Or, there could be some crud lodged in the cartridge, or (if it has one) the inlet filter screen could be clogged, reducing the flow.