Two things can slip, and the load tells you which
A crank rack is a drum, four wires, and a ratchet that stops the drum turning back. The ratchet is one small spring-steel pawl dropping into the teeth of a wheel, and it is the only part in the whole assembly that wears: the pawl's tip rounds off, it starts riding over a tooth instead of catching, and the drum turns backwards under load. Everything else — drum, wire, rails, ceiling plate — is essentially indestructible in normal use.
So the question is whether the slip is at the ratchet or at the drum, and the rack itself answers it. Wind it with nothing hanging on it. A crank that slips when empty is slipping at the pawl: there is no load on the wires at all, so nothing about the wire can be responsible. A crank that winds smoothly empty and slips only when loaded is the wire — it has piled up on the drum and is riding over itself instead of lying in one layer.
Why winding harder makes a wire fault worse
The drum is narrow and the wire is stiff, so it only winds properly if it goes on in one orderly layer, side by side. Let it cross once and the next turn lies on top of the crossing, then the next on top of that: a heap builds at one end and the effective diameter grows, so the crank gets heavier, then suddenly light as the pile collapses and the wire jams or snaps. Continuing to wind is what turns a loose coil into a jam, and it is the natural response to "it feels stiff".
The cure is entirely counter-intuitive: run it all the way down until the wire is off the working part of the drum, then wind it back up slowly and evenly, guiding the wire across with a finger so each turn lies beside the last. If the pile has already jammed hard, do not force it — the wire will kink, and a kinked wire cannot be straightened well enough to run. And the habit worth dropping: winding to the top and then giving it two more turns to be sure. Those two turns do nothing for the washing and everything to wear the pawl.