Claire and Deborah,
I thought the video (or maybe another video I recently watched) said do not overmix as the mixing process will start developing the gluten which you want in bread but not cookies. The point is maybe mixing more is not correct.
Found this on google about being dry. Dry – “Dry” or “Crumbly” dough is a product of over-mixing or using too much of any ingredient during the mixing process. This can be reversed by adding one to two tablespoons of liquid (water, milk or softened butter) to your mix.
I am doing a test batch later this week so reading all the comments!