There are several major causes of sticking:

Differential or Wall Sticking is a condition in which the drill stem becomes stuck against the wall of the wellbore because part of the drill stem (usually the drill collars) has become embedded in the filter cake. Necessary conditions for differential-pressure sticking, or wall sticking, are a permeable formation and a pressure differential across a nearly impermeable filter cake and drill stem.

Borehole Cave-in is a result of wellbore instability. The instability is classed as either mechanical or chemical.

Chemical wellbore instability or shale instability is a resulting lack of shale strength as it is repeatedly exposed over time to drilling fluids where a water phase is present.

Mechanical well bore instability is the caused by using mud of insufficient weight whereby creating excessively high hoop stresses on the well bore wall. Any increase in borehole inclination increases the risk of mechanical instability. Encountering unexpected layers of very soft sand or filter or hard rock can cause cave-ins.

Drill string failure can result from drilling with a high or a low degree of structural stress on the drilling stem and tools. Inadequate pipe strength or quality coupled with many directional drilling techniques can result in twist-offs and loss of the entire assembly.

Drill Bit Jamming is when the drill pipe and bit may become jammed when the drilling fluid is not allowed to thoroughly clean the borehole prior to stopping to add another joint of drilling pipe or the fluid is too thin to lift gravel from the bottom of the borehole.