First off, shortbread is all about that perfect balance of buttery richness and crispiness, so let's make sure we're on track with the basics. One common culprit for chewy shortbread is too much moisture in the dough. Make sure your butter is cold and firm when you start, and try not to overmix the dough to avoid incorporating too much air and moisture. Lastly, don't forget about the thickness of your shortbread dough. Thinner dough tends to crisp up better than thicker dough, so try rolling it out a bit thinner before cutting and baking. 
