The wort may not get sour if the kettle souring process is not followed as it should; another reason for not getting a sour wort could be that the wrong bacteria strain is being used. If you follow the kettle souring process correctly, souring should start after 12 hours.
If you do not notice a pH drop within the first 12 hours after inoculation with bacteria, you should check the temperature regulation. Ensure you maintain wort temperature at 80 to 100 degrees Fahrenheit. Raising the temperature a little above 100 degrees will further stimulate the proliferation of the Lactobacillus culture.
You can also reboil the wort to kill off every other organism. Ensure pH is at 4.0 to 4.3, and you can acidify with lactic acid if it is higher than this range. Then inoculate fresh strains of the bacteria, preferably a starter culture.