Also, have you checked gravity or looked for a kraeusen ring? It might be fermenting but CO2 is leaking out the lid or stopper instead of going through the airlock.
I'll add this - If you do extract batches using top-off water after the boil, both the boil water and the top-off water need to be treated with campden. While waiting for the boil to finish, the campden-treated top-off water will need to be kept sanitary since it won't be boiled. All this is...