When you say "3/4 priming sugar", what exactly did you use, and how much?
What temperature have you been storing the beer at?
Just some info :
You should use 3/4 of a cup of corn sugar per 5 gallons.
Normally (please note "normally) the definition of priming sugar IS corn sugar (Dextrose).
You can also use DME (Dried Malt Extract), at a rate of 1 1/4 cups per 5 gallons. As well as lots of other sugars.
In any case, you should let it sit for at least 3 weeks at 70°F.