In each series of seven, there is an interval between the sixth and seventh judgment (The Design of Revelation, pg. 6).
“The judgments on the world are complete in six: after the sixth seal and the sixth trumpet, there is a pause. When seven comes, ‘there comes the kingdom of our Lord and His Christ.'”
Jamieson, Faussett and Brown’s Commentary, “Introduction to Revelation.”
“The intervals of all three predicatory schemes (Seals, Trumpets, Bowls) predict historical events which are altogether religious in nature affecting the course of human events.”
Fred P. Miller, Revelation: A Panorama of the Gospel Age, Ch. 9, “The Seven Last Plagues.”