A story is constructed out of moments of tension and release. Yet, they may not appear in that order.
A story may open with a lack of tension, and the tension occurs in the latter half of the story. Some stories even appear completely absent of it.
But tension is always there, lurking in the corners.
It could be the calm after a storm or hovering beneath the surface of the story. It may be in the unspoken interactions of the characters, or it is even more meta and feeding off the tensions of the readers expecting tension to occur.
Whether the tension is present or not, know audiences expect it in all stories, whether you put it there or not. And they will apply their own tension if they don't find it.
So whenever a story seems absent of tension, search for where it may be. It might not be occurring on the story's surface, but it's there and ready to ruin even the loveliest moment of calm and release.