How the rule works
The number of corners on the figure increases or decreases by one for every cell: triangle, square, pentagon, hexagon. The rule can run to the right, downwards or both. The circle is not included; it has no corners to count.
It is a step by step change in the shape instead of the number. It is missed because the shapes look different rather than like a series, and because the difference between six and seven corners is small on screen. Count, do not guess.