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. 