The most common reason for poolish not rising is a dead or underactive yeast. To test the viability of your yeast, place a teaspoon of it in a small bowl of warm water mixed with a teaspoon of sugar. If the yeast does not start to bubble and foam after 15 minutes, it is most likely dead. Another possibility is that your poolish is too cold for the yeast to begin activating - in this case, try leaving the poolish at room temperature for 1-2 hours before putting it in the refrigerator to cold ferment overnight.