Recently I installed a new kitchen in a flat in a very old house (1830s) in a neighbourhood known for sagging pipes.
The old pipes up to the soil stack were cleaned by a pro, after the kitchen was installed. Naturally the sink got blocked (only draining very slowly). Plungers were applied and as last resort some chemicals. Nothing worked.
After doing dishes by hand for a while (2 weeks), rather than the dishwasher, the sink is magically free again. Is it possible that this is just delayed reaction to the chemical drain cleaner, or was it the increased flow from the manual washing that did the trick? I have never seen a sink unblock itself before.