Making shortbread thins

Dear Lindsey,
Semolina Shortbread,
I made the shortbread according to the recipe, but as I wanted thin biscuits I rolled them out with a rolling pin. The result was excellent, but I had some initial difficulty with the dough, which was too crumbly to roll out, even after chilling. I reshaped it, sprinkled a little flour and tried again. It worked, but it was still very crumbly and prone to breaking up. However, once cooked they were perfect.
I would like to know what is the recommended thing to do if the dough is too crumbly and starts to break up when rolling it out?
Yours, Paul