Slow Cooker - Why The Water Not Boiling Even After A Long Time?

The water is not supposed to boil, but simmer. It is a slow cooker after all.

If you use room temperature water to cook inside, it will take far too long for the food/water inside to boil. So, don't use room temperature water, but use hot/boiling water to cook inside the slow cooker instead.

Also, please check if you've accidentally set it on preset mode. Preset means that the slow cooker will only start cooking after the timer has run out.