Probably the paper itself.
Try Fanning it
Keep the paper in its packaging (unopened) as long as possible. If you don't go through a lot of paper, but you take out a couple reams and load them in your copier or printer, they will be more likely to stick or cause jams than if you leave reams unopened until you need to use them.
Use good-quality paper. Higher-quality paper is typically more expensive, but it runs through a printer so much better. Some good brands of paper that in my experience don't jam as much as other brands include HP,
Xerox (No longer exists), and Hammermill.
Monitor the humidity. Paper stored in a very humid room will be more likely to stick together. Especially in the summer, when it's hot and humid, do whatever you can reasonably do to prevent excessive humidity. Some larger printers offer tray heaters, which can also help with paper sticking together.
Flip the paper every now and then. Some printers have flat paper trays, but others only have partial bottoms, which causes the paper to bend or droop, which can lead to jamming issues. Every so often, take the paper out, fan it, and put it back in upside-down, to help even out any drooping in the paper stack.
We have a Xerox 8860 Color printer that will not stop jamming behind the front cover (picture below) it looks like it is not able to feed up into the exit area. I have tried cleaning all the rollers and sensors and the troubleshooting tips from this website to no avail. Any advice would be greatly apprciated.