First of all, I apologize for my english. Im colombian, spanish speaker, so some words might sound weird, but I hope you can understand my point.
Second of all, I am not a pro player. I discovered the game around 10 years ago, I loooove it, but just this year I found out that there was a community dedicated to enhance it with mods and many more, I used to play the Vanilla game (I didn't even know it had a name LOL), but now I added the NAM, it is very useful. Apart from that, I still play a Vanilla game.
Ok, so to the point. I have 12 city tiles running now, 5 or 6 are fully developed, the other ones are on their way. I've seen in some cities that some buildings (specially commercial, but it happens in every type of building) kinda "drop off" their wealth, so I had a beautiful shiny C$$$ tower, but in a sudden, it becomes dirt and C$$. I am a skyscraper mayor, so I don't want to see that kind of things happening. Is there any way to mantain the wealth of the buildings? What kind of things should I be aware of to keep my towers all shiny and clean? I see this is not just a problem of traffic because it can happen in an important road or avenue, or in a district away from everything. Oh, and my RCI shows the 3 residential bars totally green (in fact I tried to create fully residential cities, and it is not easy, they want to work crossing the street, I always end up mixing things)
I added images of my region and an example in one of my cities. Hope you can give me some advice!
Wish the best to all of you, keep safe in this difficult times!