If your footstraps are twisting with the anti-twist plates installed, about all you can do is to take the take the footstrap off completely by unscrewing both screws, and then check the condition of the anti-twist plate and the oval toothed SS washer.
Be sure the pin or tang on the ant-twist plate is still OK.
Look at the oval toothed SS washer to ensure that the tooth part still extends beyond the middle (where the screw compresses it).
If any are damaged, then need to be replaced.
Then clean everything up and apply a bar of soap to the screws (maybe even slice off some "slivers" of soap and push them down into the FS screw hole in your board.
The soap serves as a lubricant and makes getting the screws nice and tight much easier.
Also be sure to use a good quality #3 Phillips screwdriver (something with the black oxide on the tip (indicates the tip is hardened, has good geometry, and has been protected from rust by the black oxide coating) is usually the best).
The re-install your foostraps.
If your footstraps aren't in established pairs/sets, then think about moving the back straps to the front and the fronts to the back so the anit-twist plate "points" will stick into a different part of the footstrap.
After you get them installed nice and snug, go out an sail for half an hour.
Then come in and retighten the FS screws again.
Now they will stay in position for quite a long time.
Just check the tightness every few sessions as they do tend to compress the materials under the screws over time.
Another trick is to add a flat washer on top of the oval toothed SS washer to spread the load out over more of it's surface.
Hope this helps,