Create secondary characters who bring new tensions to the story.
Introduce new problems.
Give a character a complicated history or situation.
Create obstacles for your hero.
Complicate things.
Remind the reader of the stakes.
Find ways to keep your protagonist moving from one location to another.
Add time pressure, like a ticking bomb.
A plot is also a narrative of events, the emphasis falling on causality. 'The king died and then the queen died,' is a story. 'The king died, and then the queen died of grief' is a plot. The time-sequence is preserved, but the sense of causality overshadows it.
10 Tips for Plotting Your Novel: Step-by-Step Guide
