You may have cooked the butter and sugar over higher heat and for longer than appropriate. I have a recipe from Brennan's that calls for stirring constantly over low heat until the sugar dissolves.
If you cooked the mixture with the rum for too long before trying to flambé, some of the alcohol will have cooked off making it too weak to ignite.
This is a quick dish. You should not be cooking for long periods.
Edited to add: Same recipe as what merstar posted.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan