It sounds like your ADF unit is beginning to fail.. It may need to have its wheels or rollers replaced.
However, you may just have a jam or debris inside the ADF.. So lets start by watching this video for a similar model.
At the two minute mark, it shows how to get to the wheels and rollers of the ADf that actually pickup the paper..
Inspect thoroughly to make sure there isnt any paper jams. Also, inspect for any debris.. Now, for a bit of maintaince. You will notice rubber wheels that actually pick up the paper.. Take a lint free cloth or a cotton swab.. With the machine unplugged from power.
You want to wipe clean those rubber wheels.. Spin them and wipe them and clean them, around the entire outside of it.
Once those wheels are clean.. Put the machine back together.. And then see if it can feed more then one paper at a time.. If it cant, then the adf rollers are failing.. You can either spend money repairing it, or use it one paper at a time or else just use the scanner bad instead..
Let me know if you get stuck or have any issues, and if you cant figure out how to clean the wheels let me know as this is a very important step!!
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Senior Technician and Network Switching Systems Operator / Maintainer.