Why is my casing pulling away from the sides?

Casings will oftentimes "shrink" during the course of colonization, and will often do so towards the end of a flush.
This has 2 main reasons: Growing mycelium converts the nutrients in the substrate to mycelium, heat and CO₂. It has been established that an oyster mycelium converts up to 50% of the substrate to energy and mushrooms during its lifetime. This substrate is then "gone", that's why the substrate amount is getting less over the time. The second reason is the substrate pulling its water resources together and providing it for the fruiting stage. If, for example, 100 grams of fresh fruit are picked from a casing layer, that means the casing has lost at least 90 grams of water to the fruiting stage. The loss of water is evinced by the "pulling". A pulling -away casing can often be easily rectified after a flush is harvested: simply by adding more casing material to the sides where it has pulled, patched any casing layer that may have been pulled up when you harvested (on the ends of the stems), and mist with water. The key is to always provide the casing layer with water by misting it so it stays moist, but it shouldn't be wet. Water is key to the hobby, but too much can drown!