After reviewing your wiring layout, you may have too many drops on the lower level from the red TIU channel. On my layout, I experienced significant signal strength degradation when I had too many track sections (wires from a single distribution block) feeding off of one TIU channel. 13 sections may be OK, however I try to keep the maximum number of sections to 8 or 10. You may not have this issue, however if you do, you will only need to run wires from the blue TIU channel to the track sections nearest to the blue distribution block that you currently have on your red channel.
There is no need to have only one TIU channel powering the lower level, it can be 2 channels. If you have problems you can simply add another TIU channel, or just share the extra capacity on the upper level channel with the lower level.
I have 4 channels working on my layout, with about 8-10 drops per channel. When you wire the layout and perform the DCS signal strength test, any signal strength issues will become obvious. If they occur on the bottom level, then simply disconnect some feeder wires and see if the strength improves on the remaining sections. If it does, then simply move some track connections from the red to blue channels.
good luck with your layout build,
