SMASH Is Slow Goin' On The Fermentation

Hey all, I brewed a SMASH Maris Otter/Chinook on 3/22/13. I used US-05 with a starter, yeast nutrient and aerated with pure co2. My OG was 1.050 and I still have it in the primary because the blow off is still bubbling about once every ten seconds. I just checked my gravity and after almost three weeks, I'm only at 1.024. The sample tasted pretty good, so I'm wondering, do I need to throw some more nutrient in there? Is this just the nature of US-05? Fermentation has been good and steady but I just don't want my yeast to tire out. What do y'all think?

 

What was you mash temp and thickness? You could have produced less fermentables. Starter for us-05? Just rehydrate or dump normally.
3 days of gravity readings, no change = done fermenting.

 

You aerated with pure CO2?

 

If I had less fermentables why would my gravity reading still be so high? doesn't the gravity reading measure the sugars in a solution? Also I make starters normally so I figured it couldn't hurt.

 

I'm an idiot I meant o2