Four (4) main factors to consider include moisture content, temperature, particle size (and shape), and time at rest.
1) An increase in moisture content will generally make solids more “sticky”. Some solids will absorb moisture from the air, which is why nitrogen is often used as a carrier gas (among other reasons).
2) For some solids, their ability to flow can be adversely impacted by temperature or even the length of time that the particles are exposed to a specific temperature. For example, soybean meal flows nicely at 90 °F but start to form large bridges at 100 °F.
3) Generally, the finer a bulk solid becomes, the more cohesive the particles. Round particles are generally easier to handle than “stringy” or oddly shaped particles.
As particles rest in a bin, they can compact together from their own weight. This can create strong bonds between the particles.
4) Often times, re-initiating flow can break these bonds and the solids will flow as normal, but this can depend on the load at given locations in the bin.