Screw spinning freely – no longer gripping
The screw hole has stripped: the wood around the screw no longer provides any grip. The simplest fix: push one or two toothpicks or wooden skewers with a little wood glue into the hole, snap off the protruding ends, and allow to dry for at least 30 minutes. Then drive the screw back in at the same spot – the wood fibres will now grip again. Alternatively, use a slightly thicker or longer screw. For load-bearing structures (shelves carrying weight), also check whether the material is fundamentally suitable.