[FrankJ]

Chain stretch is a slight misnomer. The links don't stretch. Or more accurately the load needed to deform the links is very close to its breaking load. What you are seeing is wear in the pin & bushings. During break in the high spots wear quickly which gives you the rapid initial stretch. You take a length of worn out chain and lay it on a flat surface and push in & out you will see a lot more movement than in a new chain. Sprocket wear also looks like chain stretch.

Ratcheting is caused way too loose chain, not enough chain wrap around the sprocket, or the sprockets being in adequately supported (allows the CC distance to shrink under load.