I just tried an outstanding turtle recipe. The caramel is brown sugar, butter, corn syrup, salt & sweetened condensed milk heated to 248 degrees. They started out great nice & chewy, not too hard BUT after a week the caramel is turning grainy, sugary & crumbly. What happened?
The sugar is crystallising. Try replacing the corn syrup with glucose syrup, or adding a little cream of tartar, both of which prevent crystallisation.
Not to point fingers, but are you sure you didn't leave out the corn syrup? Even a small amount is pretty effective in preventing crystallization of table sugar. The glucose in corn syrup gets in between the sucrose crystals and interrupts the crystallization process.