You may not have heated the solution enough. If there are any undissolved sugar at the bottom or sides of the glass, that's where the crystals will preferentially grow. Also, I have found it is helpful to drag the string in a little bit of sugar before hanging it - this way you seed the string. Although, this may result in smaller crystals.
Other things that may impact the ability of crystals to grow include the purity of your sugar, the purity of the water, the cleanliness and smoothness of your glass, and the material of which the string is made. Crystal growing is a very finicky process.