100w should be sufficient for a 29g tank in a room temperature environment.
first, check that it's getting power from the outlet.
second, check the thermostat.
third, check that the heater actually gets hot (you can do this by BRIEFLY removing the heater from the water after it's been on for a while, it will steam the water off.. be careful not to leave the heater out of the water for more than 5-10 seconds, as it can damage the element)
lastly, if all above is checked, get a new heater.