Hi Guys,
Setting of stains can certainly occur with heat, rinsing blood with cold water is an example of this.
Partial dissolution of the stain with cleaning products, which then make them more fluid, often with agitation, rubbing etc will allow the stain to bond much more effectively and consequently be more difficult to remove, lily pollen isn example of this.
There is also the pH issue where some stains are more difficult to remove if the pH has been made more alkaline, red wine removal is an example here.
Cheers
Doug