Recycling trash cans double before my eyes

First, Sims need to be educated to recycle. So if your education level has dropped, your recycling may slow or stop.

Make sure your recycling center has metal or alloy additions, and that they are working. You have to have a trade depot or trade port to get rid of the recycled goods. Try to plop them close to your entrance so you can get a good fast traffic flow. Make sure you have a storage facility for each type of recyclable you're making at the recycling center. 

The flow should go like this: Double-click the recycling center to edit the facility; add metal processing units and max out the delivery trucks. Exit and plop a trade depot next to the recycling center. Edit the depot, bulldoze the freight warehouse that came with it and add a metal warehouse. Exit the edit and click on the manage button and set metals to export.

Now, the recyclables should get picked up, dropped off at the recycling center where it processes them and turns them into metals. The recycling center then transports (via the delivery trucks) the metal to the trade depot. Regional trucks will come into your city and pick up the metal, paying you for it.

Garbage dumps, recycling centers, trade depots and trade ports sometimes but not always get to a point where the trucks stop going out to collect (or deliver goods from trade depots and ports). It's been a bug that's been present since the launch of the game. If and when this happens, demolish the trucks and add new ones and be prepared to have to do it again.

Community colleges and universities with regional traffic will cause you to lose city students over time, causing your education level to drop. It will count these missing students as "uneducated". This is another bug that's been present forever. This can be a big annoyance to fix; I would recommend you watch Skye Storme's video https://www.youtube.com/watch?v=ThgobF98QHI on how to fix this.