Help for Heirloom Cookies?

I tried making these for Christmas; I bake them once a year and sometimes they come out perfectly and other times, like this one, the dough won't stick together. I have to add a little milk to roll them in my hands - and that makes them a bit tougher, rather than melt-in-your-mouth flakey. Any ideas about what makes the difference? Refrigerating the dough made it even worse!

Mother’s Cookies

Cream ½ cup butter, ½ cup shortening (add gradually) and ½ cup sugar.
Blend in ½ tbsp almond extract and 2 1/4 cups sifted all-purpose flour.
For best results refrigerate dough before using; keep second half refrigerated until ready to use.
Shape with hands into very small balls.
Bake at 350º F. for 8 to 10 minutes.