less heat LOSS
a thermocouple would tell you quickly what temperature you are reaching. You can buy some thermocouple wire for around $10, measure the voltage with a cheap multimeter and then use the thermocouple response charts to see what temp you are at.
Once you have enough heat...
What type of aluminum are you starting with? If its little bits, the oxide layer on the surface will tend to hold the stuff together and make it look like it's not melted, even though it is inside. If its bigger pieces, you will have trouble getting enough heat transfer from the crucible to the aluminum, as only the corners of the block will be touching. It would be best to have a furnace where the whole atmosphere is at temperature, not just the crucible. That will help greatly with melting.