I have observed this also. Although I don't know the exact reason, I suspect it's for two reasons:
a)some of the more volatile liquids evaporate.
b)oxidation of some of the emolients probably solidifies some of it.
Of the two, I think a is the smaller and b the larger.
Liquid soap is a viscous (thick) solution that can form a solid by the evaporation of water in it. Some liquid soap remains at the tip through capillary action and forms a solid deposit by the evaporation of water. Initially, the spout opening becomes narrower and eventually clogs up.
Some of the scientific concepts involved are vapor pressure of a solution, rate of evaporation, surface tension, capillary action, and viscosity.
There is a more complex answer to this. Many modern soaps used in soap dispensers are not true solutions, i.e. a material dissolved in water. They are a mixture of materials to give the pearly, homogeneous appearance. In some cases a type of microemulsion. ((Broken Link Removed) This mixture has been chosen to have a certain viscosity so that when pressure is applied to the pump it is easy for soap to be produced from the nozzle. This microemulsion represents one pregion of a complex phase diagram and if the local conditions change (colder/warmer) it may not stay as one phase and separate into two phases and may even deposit a solid. The overall composition has stayed the same, but the mew mixture of phases will be more visous than the original. Indeed if a solid has been deposited this may block the nozzle.
This change of behaviour can also be brought about by one of the phases evaporating as this may cause the microemulsion to separate and the result is a more viscous mixture that is difficult to pump.
Thank you so much DrBob222, GK, and Dr. Russ for your very satisfying answers! Too bad there isn't a solution, so I may have to revert to a solid piece of soap again...
Dr. Russ, the dispenser didn't come with soap. I use it to hold a liquid soap called "Dr. Bronner" made with non chemicals or animal derived products, as well as not tested on living beings. The entire one quart bottle is covered with uplifting spiritual quotes...These bottles also have a nozzle on the cap, but the soap doesn't solidify in them which I consider quite strange.
Again, thanks ever so much for your thorough answers. :o) Pietra