Why does my tamale masa keep cracking when I spread it?

The masa is too dry. For tamales, the dough should be significantly wetter and softer than for tortillas — it should feel like thick, spreadable peanut butter, not like a dough you knead. The classic test: put a small ball of masa in a glass of cold water. If it floats, it has enough fat and air. If it sinks, knead in more fat (lard or vegetable shortening) and add a little more warm broth until you get there. Crack edges are almost always too-dry masa.

The float test is the traditional method but I also use the squeeze test: take a golf-ball sized piece and squeeze it. If it cracks when you squeeze it, it is too dry. It should compress without cracking and should be slightly sticky. If you get to cracking edges you are well past the correct texture — add warm liquid in small increments and re-test.