Brown sugar makes brittle cheesier. I would use white sugar and if it still doesn't work try a different recipe.

My dd had her heart set on making her teacher peanut brittle for Christmas. We of couse waited until the last minute and did the microwave version. It looked good and broke nicely after we quickly chilled it in the freezer. But the next day it was chewy and stuck together.
So this week we made it on the stove with a candy thermometer (got it up to 302 hard crack) let it cool on the counter and it is chewy!! It tastes good but is hard to actually eat.
Anyone have any advice?