Hexokinase specificity?

I ran an enzyme specificity assay by testing if hexokinase would dephosphorylate ATP to ADP using six different sugar substrates. Currently I am writing my discussion and I cant figure out why hexokinase works for some sugars (clucose, fructose etc) and not for others. What determines the specificity of hexokinase? Does it have to do with the number of carbons (maybe) I'm stuck please help!