The suspension of clay particles (after initial settling has occurred) is often due to the type of particle and its charge. Different chemicals have different charges and the right mix may cause the charges to offset and thus fall out of suspension. See the above link. Here is part. It lists more causes , reasons and info .

The chemistry of colloidal clay suspensions

The chemistry of colloidal clay suspensions is not completely understood, primarily because fairly complex physical and chemical processes are involved. Clay particles are extremely small;
some are even smaller than bacteria. Therefore, they will not settle readily, even in still water. The small size of these particles means
that they have an extremely high surface area relative to the volume of the particle. A clay particle can be envisioned as a flat plate cov-
ered with a negative electrical charge that attracts the positive ions in water. Positive ions that are immediately adjacent to the clay particle are said to be "adsorbed," while others that are farther away are less strongly attracted. In water, negatively charged clay particles are surrounded by clouds of positively
charged ions. When these particles, surrounded by their ion clouds, come close to each other they are repulsed, much the same way similar poles of two magnets will repel each other (Fig. 1). The cumulative effect of the repulsion
of a huge number of small particles prevents their aggregation into larger, heavier particles that would settle more readily. Taken together then, the extremely small size of clay particles and the surface electrical charge explain how particles remain in suspension.

Flocculation and coagulation

Flocculation is a way of controlling clay turbidity by adding substances to water that facilitate the formation of bridges between particles (Fig. 2), allowing them to combine into groups of small particles called "flocs" (Fig. 3). Metal salts make good flocculants, depending on pH. These hydrolyzed metal compounds
destabilize colloids by shrinking the layer of positively charged ions surrounding clay particles, which increases the attraction of one particle to another (coagulation). Hydrolyzed metals also can be adsorbed onto the surfaces of clay particles and create bridges to other particles (flocculation). As these particles begin to settle, they ensnare other particles, become progressively heavier, and settle much more readily from suspension. In general, the effectiveness of coagulants increases with the
charge on the metal ion. The sodium (Na+) in sodium chloride (NaCl) is not a very effective coagulant. The calcium (Ca2+) in gypsum (CaSO4) is more effective because it carries a +2 charge.The aluminum (Al3+) in alum and the ferric-iron (Fe3+) in ferric sulfate are more effective yet because they carry a +3 charge. Some companies now manufacture various synthetic "polyelectrolytes," which are large, long-chained molecules with even more charge than the metal salt coagulants listed here.