1. Garbage and recycling. If you are getting garbage trucks from other city and they dont reach your city in time (it happens a lot, they turn around at the entrance of your city, and then never come back) then there is an accumulated garbage and recycling that is not deleted from the "memory" of the game.
2. Trade depots. Sometimes, unexpectedly, trucks from trade depots go outside city being lost in the traffic jam. Other times they go from one depot to the other, just follow them closely and "perhaps" you will get them working. I had this problem once, I built another depot and from them on everyting went fine.
3. I have some ideas about schools, but I think that part of the simulation is broken.
As you can see everyting is related to traffic, not only in your city but in the highway outside.
This game seems a gamble game. You have some chance to get it working as intended (0,0000001%).
I don't think anyone at EA has ever play/test this game.
Regards