I have a tart recipe that has a shortcrust base that I have done several times. On the first attempts, everything was okay with the base - the "dough" was crumbly but could be rolled and kept its shape relatively well.  Now I tried making it and the "dough" turns out very crumbly, more like dry flour than dough. Rolling is impossible, and only after a lot of compaction with my hands was I able to make something workable.  The ingredients are:        300 g flour   100 g sugar   200 g butter, cut into small cubes   1 beaten egg      The instructions are:        Mix flour, sugar and butter   Add beaten egg and stir until combined   Flatten the dough, wrap in cling-film and place in refrigerator for 2 hours      The instruction imply to use a food processor, but I have been using my KitchenAid stand mixer with the flat beater. I also tried mixing by hand (with a fork as well as literally by hand), with minimal improvements.  What am I doing wrong now, and why did it work before but not anymore? 
If your dough is very dry and crumbly, it needs more water.  Add a few mL to the dough when adding the beaten egg. 