You may have some corrosion on the threads. if it's the first time removing the screws in a long time, and it caused the nutserts (or sometimes called a rivet nut) to loosen. If you haven't done it already, spray a little penetrant on them, and let it soak for a while. Then I'd give Grumpy's method a try again.
When you get them off, the nutserts can be replaced with a kit similar to this, depending on thread size.
https://www.harborfreight.com/45-pie...-kit-1210.html
I had to replace two on our upper awning brackets when we replaced the fabric.