Why does my rigging go slack a few weeks after I finish a build?

The droop you're seeing is quite common when using stretchy materials on long runs, especially those spanning considerable distances like from the pagoda to the funnels. It's rarely just the glue failing, though initial bond strength matters. Those materials, even if rated for stretching, have a degree of elastic memory, and over time, they naturally want to return to their unstretched state, or they may simply relax under tension if the anchors aren't rigid enough. Your ten seconds with the CA was good, but it doesn't create infinite strength. You need firm, non-yielding connection points to keep the tension in the material itself. Heat is definitely a factor to consider if your shelf is near a window. Temperature changes and humidity fluctuations can affect synthetic fibers and thin wires, allowing them to settle or change their internal tension. If the shelf gets warm, the material expands slightly and then loses some of that stored tension when it cools or stabilizes. The solution for long-term integrity is usually not just re-gluing, but actually increasing the anchor strength and securing the material further. Instead of relying purely on the dab of CA at the connection, try running a thin thread or a fine wire through the attachment point and securing that with a tiny bit of epoxy or superglue. This adds mechanical support to the anchor point. As for re-tension, it's something a lot of modelers do, usually after a couple of weeks if they notice significant sag. It's much easier to adjust while the model is still clean than to strip it all back out. If you need to re-tension, just pull gently on the lines and add very tiny spots of adhesive to the anchor points. You might find that a tiny bit of clear epoxy, rather than just CA, holds the strain better over the long haul.

If you decide to redo sections, try using a higher quality, non-stretch material for the absolute longest runs where the slacking is most dramatic. Sometimes those stretch wires just aren't designed for spans that long even if they were drum tight initially. Consistency in anchor quality is the real trick for keeping everything tight over the years.

Agree with the point about the sun being a real factor here. I had the same thing happen on my Bismarck, the long runs went slack over a summer and I eventually figured out the afternoon heat on that shelf was the culprit. Moved it to a diferent wall and the rigging has stayed put since. I also went back with a tiny drop of epoxy on the worst anchor points like suggested and that did hold better than my original CA alone. If you redo the sagging runs, wait for a cool part of the day to tension them so you are not pulling them tight while warm.

On my Kong I went through the same cycle of tightening and re-gluing before I gave up and re-rigged the whole thing with stretched sprue. Melted a bit of clear sprue, pulled it out thin, and let it sit a day before stringing. That stuff has no elastic memory at all, so what you tension is what you keep. Took me a few failed pulls to get consistent thickness, but two years on and not a single run has drooped. Might be worth trying on just the two long runs before you strip everything.