1. The distribution of sampling points is uneven and unreasonable, such as being mostly distributed around the coal pile, the edge of the carriage, and on both sides of the belt, resulting in the collection of large blocks of coal. 2. The width of the feeding opening of the sampling tool (or sampler) is too small, and large blocks of coal cannot be collected. 3. The quality of the sub samples is too small, and the particle size composition of the collected sub samples cannot represent the particle size composition of the neighboring coal. 4. The number of sub samples is too small, losing the representativeness of coal. 5. The sampling periodicity coincides with the fluctuation period of the coal quality being mined, resulting in unidirectional deviation of the coal quality of the collected coal samples.