Whoever install the 3-section folding stairs to my attic, or the manufacturer, managed to make it so that the final section, when it folds out and contacts the floor, is separated from the middle section by about 3/8". Thus, when you step on the contraption, it bends passed where it should be, putting all the stress into a single line of contact with the section above it; it's hyperextended, like - we hope this never happens to you - an elbow or a knee.
I have temporarily glued 2 pieces of wood, on for each side, to the lowest step section, and this kind of works. It still allows a little play.
Replace it. Nice ones with metal ladders and doors that seal are not expensive. I got one at HD for just over $100 and R&R by myself only took about 2 hours, probably less with a helper. So much safer!